Ideas on best bang for the buck..

Well, specials change. And I don't know what's available to you in Costa Rica.

You might be able to get a better quality PSU + case for cheaper. See if the Sonata III cases compete in price with the Thermaltake V3s. They have a higher quality power supply. Otherwise, depends on local pricing. Here in the states we can get an Antec One case and Antec Neo Eco 450c (again, the benefit being higher quality power supply) for $75, but they're both on special at the moment.

For processor/mobo you might look into whether an i3-2100 or pentium g850 + h61 mobo is cheaper. Since I assume you're not going to be overclocking those Phenoms, they should provide better gaming performance. Here in the states the totals should come to roughly $170 for an i3-2100 + h61, and a pentium 850 + h61 comes to about $130. I'd look into those option for price/performance. If the phenoms are cheaper than the i3+H61 in Costa Rica, then there might be a good argument for them.

6870 generally hits a good price point. You might also consider buying up EVGA's B-stock 460s if the EVGA store will ship to you ($90, ) or even bumping up to 7850s if you think higher graphics quality in games like BF3 will be needed to draw in customers (probably not). The 460s run behind the 6870s, but not by a mile:
http://www.anandtech.com/bench/Product/542?vs=540

Drops your costs a little though, dunno how tight your upgrade budget is.

If anyone else chimes in, it might be a good idea to mention which games are popular with your customers/you will be offering.

Hmm. Most of the deals I know are from us.ncix.com.

For cases, you could look at Antec One, Bit Fenix Merc Beta, Coolermaster Haf912. Those are pretty good cases from NCIX.
They're often offering a power supply deal: Antec Neo Eco 450c. I'm sure you can look up those parts at us.ncix.com and compare yourself.

I dunno how their shipping will work though. Within the U.S., NCIX offers free shipping for orders over $50, but they're an offshoot of a canadian retailer, so... might be a lot to Costa Rica?

Aside from shipping, any of those cases+ the PSU should be about the same price or less than the Thermaltake V3 and you'd have a better power supply. Note though that the PSU only has 1 PCI-E connector, so you'd have to use molex -> PCI-E connectors if the video cards you get need 2 and don't come with an adapter as an accessory (no problem with adapters, just something extra to order so everything works).

For instance:

GPU: EVGA 460 ($94) (scroll to bottom of page)
http://www.evga.com/products/prodlist.asp?switch=20
& http://www.amazon.com/gp/offer-listing/B0007RXDDM/ref=sr_1_1_olp?ie=UTF8&qid=1341974100&sr=8-1&keywords=molex+pci+power+adapter&condition=new

Best bang for buck?
I would go for used parts a lot cheaper and exactly the same
Ive done a few rigs with all used parts and had no problems

Speaking as someone who was intimately familiar with a local LAN cafe, its all about what you can get wholesale. Originally when they opened, they got a wholesale contract and picked up the best deals they had. At the time it was Core 2's and 8800GT's and they were a complete steal. The cafe only opened with 10 PC's but it expanded to 20-25 as time went on. Sometimes they bought extra parts either for themselves or to sell on ebay (not sure if that's strictly legal) just to make the contract worth it.

As for CPU, I think I'd recommend G850. Seems gimpy I know but you need every cent you can get to go to graphics so you won't have to upgrade as soon.

edit: Sadly the cafe has since closed due to bad financial management (tried buying out a restaurant next door and expanding, didn't work out)
Edited by crashnburn_819 - 7/13/12 at 5:54pm

Before you look into builds, I think you'll need to consider what games people will be playing, then gear the computers towards those. If you have a gaming market in mind, you can adjust the PC budget to offer the best performance for those games. For example, BF3 is largely dependent on GPU, but a RTS like starcraft 2 is CPU dependent. SC2 can get away with a $100 or less graphics card and play perfectly fine at 1080p, allowing you to spend more on the CPU. For BF3 however, you could get away with a cheaper CPU then spend more on the GPU to get the higher frames for its intense graphics.

This can be hard to know in advance though. I'd look into whole sale as mentioned. Chips wise, I'd recommend the G850 or i3-2100 as well. I'm not sure if the phenom II line is still widely available or whether AMD is pushing out only FX chips.

You need to nail down a specific performance level first. You need to figure out exactly what games you are going to build for. The other suggestion I have is to examine your business model very carefully. Most cafes fail, and computer cafes usually fail and you lose more money when they do. How is the revenue stream generated with the Gaming rigs? It would take the profit from thousands of cups of coffee just to break even on a $500 rig, and how long is the lifespan of these computers? 2 years? Maybe 3? If these are being used enough to actually turn a profit they will also be taking a serious beating. I have always liked the idea of computer cafes, but every one I have ever seen has failed.