Advice for a pair of matching £1250 ($2000 US) rigs.

For gaming, right now, Intel i5-3570K is the best if you intend to overclock, or i5-3470 if you don't.
However, it's built on the Ivy 1155 platform which will be replaced sometime mid next year if that matters to you.
X79 2011 will be able to support the future Ivy-E CPUs but costs more than Z77.

To game smoothly @ 1080p, you'll need a good single-card solution, and there are many to choose from

IMHO, this represents the best performance/cost available today that meets/exceeds your gaming needs:

CPU: i5-3570k (since this is OCN) (167 £):
http://www.amazon.co.uk/Intel-Generation-i5-3570K-3-40GHz-Technology/dp/B007RUZKK6/ref=pd_sim_sbs_computers_3

Cooler: CM Hyper 212 (25 £) :
http://www.amazon.co.uk/Cooler-Master-Hyper-212-120mm/dp/B0068OI7T8/ref=pd_sim_sbs_computers_11

Mobo: MSI Mpower (170 £) :
http://www.amazon.co.uk/MSI-Z77-Mpower-Intel-DDR3/dp/B008VZROK0/ref=sr_1_1?s=computers&ie=UTF8&qid=1350740172&sr=1-1

GPU: AMD 7970 (325 £) or Nvidia 670 (310 £) :
7970: http://www.amazon.co.uk/Gigabyte-7970-Radeon-Graphics-Card/dp/B0074VVTPI/ref=sr_1_8?s=computers&ie=UTF8&qid=1350740349&sr=1-8
670: http://www.amazon.co.uk/Gigabyte-GeForce-GTX670-Graphics-GDDR5/dp/B0080DXPHO/ref=sr_1_4?s=computers&ie=UTF8&qid=1350740392&sr=1-4

Note: the Windforce 3x cooler is very quiet in operation.

SSD (boot drive): SAMSUNG 830 128GB (70 £)
http://www.amazon.co.uk/Samsung-128GB-SATA-6GBPS-Basic/dp/B007BBQPUA/ref=pd_sim_sbs_computers_38

HDD: WD Green 2TB: (75 £)
http://www.amazon.co.uk/Western-Digital-6Gbps-Saving-Internal/dp/B004VFJ9MK/ref=sr_1_2?s=computers&ie=UTF8&qid=1350740579&sr=1-2

PSU: Antec EA Green 650 Watts (70 £)
http://www.amazon.co.uk/Antec-EarthWatts-Series-EA-650-Supply/dp/B0071I55PY/ref=sr_1_28?s=computers&ie=UTF8&qid=1350740646&sr=1-28

Case: Corsair 550D (105 £)
http://www.amazon.co.uk/Corsair-Obsidian-Series-CC-9011015-WW-Mid-Tower/dp/B006L6ZSWC/ref=pd_sim_sbs_computers_18

The monitor I leave up to you to decide but I'd recommend going with an IPS panel instead of TN.

Opps, my bad.

Memory: get 16 GB cause its just sooo cheap. Sweet spot is 1600MHz C9 like this 8GB kit (you'll need 2x):
http://www.amazon.co.uk/GSkill-1600Mhz-Ripjaw-Series-Memory/dp/B005IP84DC/ref=sr_1_21?s=computers&ie=UTF8&qid=1350742147&sr=1-21

Stick with known names like Samsung, Corsair, G.Skill...etc that offer lifetime warranty.

Frankly, your budget is too high. You're looking for basically any Intel i5 (overclocking is fun but not necessary, & costs $). What I'm not sure about is your monitor. Presumably you have a single 1920x1080 monitor, but why not consider an upgrade? It'll be more worthwhile than a lot of other options. Also, for your wife's MMOs, it can be really handy to have a second monitor for desktop/browser space to look stuff up or run ventrilo, whatever.

Here's what I'm talking about. First, you just get what you need (by need, I mean for excellent performance), without over-spending as above (an Mpower motherboard? seriously?). The below is presuming overclocking isn't a priority for you, but you can do so for not too much money more. (I used scan.co.uk because I like its interface. Feel free to comparison shop the same parts on amazon.co.uk or wherever).

Core Components: 232 pounds
Most games don't take that much CPU power, but those that do prefer Intel. An overclocked i5 (quad-core) is the gold-standard for gaming, but a stock i5 is already overkill for almost all games. You won't notice a difference between overclocked & locked when gaming unless doing something wacky like playing 4v4 in SC2 all day (one of the most CPU demanding gaming tasks) or streaming your gaming. Since we're sticking with a locked i5, a B75 or H77 mobo will run it flawlessly. Going with a locked i5 means we can use the stock Intel heatsink and don't need to buy one. Add in 2x4gb of RAM because RAM is cheap but more than 8gb is pretty useless. Warning: Spoiler! (Click to show)
Video Card: MSI 660ti for 234 pounds or Sapphire 7950 for 264 pounds
http://www.scan.co.uk/products/2gb-msi-gtx-660-ti-power-edition-oc-28nm-6008mhz-gddr5-gpu-1019mhz-boost-1097mhz-cores-1344-2x-dl-dv
The 660ti is an excellent card for a 1920x1080 resolution and should max most games, high settings for an elite few hard-to-run games. For any higher resolution, you'd probably want to move up to the 7950.
(if you go with the 2560x1440 monitor upgrade below, you'd prefer a 7950. The only one in stock at scan.co.uk that's decent is the triple-wide ASUS 7950, so instead I'll link one from amazon, a well-cooled sapphire card for 264 pounds)
http://www.amazon.co.uk/SAPPHIRE-TECHNOLOGY-11196-16-20G-Sapphire-Mini-DisplayPort/dp/B0091MER16/ref=sr_1_23?s=computers&ie=UTF8&qid=1350839009&sr=1-23

Supporting Components: 304 pounds
For case, we pay a little extra to get a slightly quieter case, because that's the best thing a case can do for you: make the computer quieter. For storage we put in first an SSD for your OS & commonly used programs/games (particularly MMOs), then add a larger Hard Drive for media & overflow storage you can access at a slower speed. Add in a quality but relatively inexpensive power supply & a DVD-burner. Warning: Spoiler! (Click to show)
Tower cost (not including windows) - 770 pounds, or 800 pounds with the 7950.
You'll notice that's a lot under 1250 pounds. That's good! You can pocket the savings, having a computer more than capable of doing what you want it to do, splurge a bit on extras like overclocking capability, nicer headphones, desk chair, etc. Or improve your monitor setup, using the old monitors as secondary non-gaming adjuncts.

Possible monitor upgrades!
24" 120hz 1920x1080 monitor: Benq xl2420t (279 pounds)
http://www.scan.co.uk/products/24-benq-xl2420t-120hz-3d-vision-2-led-monitor-black-red-full-hd-hdmi-dvi-1920x1080-350cd-m2-10001-2m
Many people like 120hz monitors because the maximum framerate for games goes up from 60fps to 120fps. This makes fast-motion effects seem smoother. Some people have, essentially, worse eyesight, and the improvement in motion display will seem minimal at best to them. Also, if you buy extra glasses 120hz monitors can do 3D. Not generally considered a big deal compared to the increase in framerate. The BenQ above (along with some Samsungs) is considered the best of these. Note this only applies to the BenQ xl2420t - the xl2410 did not turn out too well. As with many monitors, color calibration is not perfect out of the factory, so if you're picky about that you may need to find presets you like for those settings. Like most monitors this has a "matte" coating that reduces glare.

27" 2560x1440 IPS monitor (at the normal 60hz): Crossover 27Q (322 pounds not including VAT and possible import duties)
http://www.amazon.co.uk/CROSSOVER-Perfect-27Q-LED-P-2560X1440/dp/B008BE6ZIS/ref=sr_1_19?s=computers&ie=UTF8&qid=1350838895&sr=1-19
Bigger screen size = bigger. Bigger resolution = sharper image despite the increased screen size. IPS is also nice for slightly better color accuracy & viewing angles than normal monitors. This Crossover is a Korean import (why? They're cheap, not 600+ pounds like standard models). There's a couple of caveats to Korean imports. Most significantly, for gaming-capable monitors you need the versions with no On Screen Display menu and only a dual-link DVI-connection for good response times (an OSD or additional connection options slow response times). Also, warranty coverage is somewhat iffy. On the other hand, you simply cannot beat the size & resolution & picture quality for the price. These are generally glossy monitors, meaning they have a bit sharper picture but need to be placed in an area where they won't reflect light into your eyes due to the mirror finish.

Long story short - go with a tower setup similar to what I have above. By only spending ~800 pounds on the tower (1600 pounds for the pair of them), you have plenty to stick in your pocket or for other computer-related luxuries. I generally recommend monitor upgrades as that extra luxury, because a shockingly high number of people have some crap low-end TN panel despite their awesome gaming towers (especially here on OCN).

Total costs with monitors:
2x 120hz monitors + 2x towers w/660ti = 2098 pounds after vat but before shipping
2x 27" 2560x1440 monitors + 2x towers w/7950 = 2244 pounds after vat but before shipping & not including vat/duties on the monitors

Personally, I'd go with the 27" monitors & 7950.
Edited by MisterFred - 10/21/12 at 10:37am

Hehe, I see what you're saying but you made it sound like I hate the guy....Dude, the OP posted this Thread on OCN, so logic dictates that he's into overclocking!
And the Mpower is one of the cheapest quality-Z77 boards out there.

Well, from my perspective there's a vanishingly small performance difference between an Mpower & say, a Gigabyte z77-ud3h unless you want to hook up multiple video cards. And don't get me wrong, most of your build made a lot of sense. I am also a big fan of the Corsair 550d and other components you recommended. The Mpower is just what popped out when I was browsing quickly.

3D capability comes with 120hz. (Actually, with 120hz & fairly expensive 3D glasses, which aren't included in the BenQ xl2420t deal linked above). So there's not a "non-3D" version of that monitor. It operates in 2D unless you tell it otherwise. If you have no monitor at the moment, the secondary monitor doesn't need to be 120hz or anywhere near as nice. A good standby for a quality 23" IPS monitor is the Dell u2312hm (170ish pounds). Like the BenQ, the Dell has excellent response times for gaming. Unlike the BenQ, it is IPS and only 60hz. I'm not sure how a Dell would look next to the BenQ. You might consider just getting two Dell u2312hms or going for the BenQ and for a secondary monitor getting some cheap 20" 1680x1050 resolution monitor.

Tough to say!

Overclocking isn't needed for max settings when gaming. In general, spending more on video card is more useful for max settings than spending on the CPU. For instance, moving up to the 7970 eXXon linked would be more useful in most games than overclocking the i5. Though again, most games won't use that extra power. Really the only benefit for overclocking in your situation will be potentially extending the life of your CPU a few years down the road. Personally, I wouldn't worry about it.

Its expected to be 15-20% faster than BD in multi-threaded apps, don't know about gaming performance (a little faster maybe if not the same).