Pawelr98 is correct, your current PSU is insufficient and you'll never need anything more than a 'decent' 400w unit with that build, even if you overclocked a Phenom X4 and the 8600GT to relatively high (air) levels, you'll not need anything more than a 350w really, if you were to unlock an X3, it'll likely need a tad more juice to stabilise the cores, so a 400w will give you nice headroom, plus room for any future low-mid-range upgrades.
As far as unlocking on your Motherboard goes, I'm afraid it's not possible. Your Motherboard has an SB700, and you'll need a Motherboard with SB710/SB750 or above to unlock any cores (even then, it's never 100% that you can stabilise the unlocked cores).
SB710/SB750 and above feature ACC (Advanced Clock Calibration), and that's the feature that enables Core unlocking.
Also unlocking is never 100% a guarantee, though it became ALOT easier with the Phenom II's, and easier still with the Phenom II's toward the end of the line (E.G. X2 560, 565 etc).
Here is a list of compatible CPU's with your motherboard though, it's a shame the Phenom II X3's are so expensive now (due to rarity and EOL), because that would be the single best upgrade you would be able to do with your current motherboard.http://www.foxconnsupport.com/cpusupportlist.aspx?type=mb&model=A78AX-K&cputype=
(It requires flashing your BIOS to the latest version, to get that compatibility)
Logic would dictate that if it can take a 95w Phenom II X3, then you could possibly pop an Athlon II or Phenom II in there too, but without 'official' support it will either work and show up incorrectly, or just not work...
In regards to overclocking your 8450, you'll likely get between 400 and 600mhz out of it at a push with a new cooler, I ran my old 8450 at 2520mhz 24/7, I think I got 2.6 out of it, but it wasn't stable and I wasn't comfortable putting too much voltage through my particular motherboard.
The Corsair CX430 is the only decent (not 'great', but better than the others there) PSU I'd recommend from that site, I fumbled my way to the PSU section and sorted by price, the 430 will handle a Phenom II X2+ GTX 460 OC combo, the CX500 will handle a Phenom II X4+GTX460 and an i5 (Sandy/Ivy) +GTX460 OC combo should you ever overhaul your rig completely, you could at least retain the PSU.
I don't know any other shops you use, so I can't recommend any better PSU's for a good price that are available to you.
As far as coolers go, the Scythe Katana 4
or the Coolermaster Hyper TX-3
are your best bet for the price, from that site, whether they'll fit okay we can't be certain of, 'cos we don't know what case you have, but they're not overly large, so should fit in a standard ATX case.
Good luck! Edited by EvoBeardy - 9/25/13 at 3:14pm