Budget Range: Around 800, my budget is flexible though.
System Usage from Most to Least Important: Gaming, Internet, School Work
Parts Not Required: keyboard, mouse, monitor, speakers, OS
Preferred Website(s) for Parts: newegg.com
Country of Origin: USA
SLI or Crossfire: Maybe in the future.
Monitor Resolution: 1920x1080
Additional Comments: Just need some ideas for a new build I'm putting together. I would like good performance in Starcraft 2 on higher quality settings, and when Diablo 3 ever comes out I would like to be able to play that maxed. The only things I'm a little unsure off is whether to go 4gig/8gig on ram and whether to go with an ATI 5770/6850. Also my power supply might be a little overkill for this build. Feel free to change any parts, I have no brand preference.
First things first. Ditch AMD. In one month, Intel's new Sandy Bridge motherboards will be re-released. The i5-2500K ($230) gives a massive performance boost over anything AMD has. The performance difference between the X4 955 and the i5-2500K is something like 50-75% at stock, and the i5 can be overclocked safely well over 4.0 GHz without breaking a sweat. I've heard reaching 5.0 GHz on air cooling isn't unheard of.
If you do choose to go the i5 route, a couple of good boards to look at are the ASRock P67 Extreme4 ($150ish) and the Asus P8P67 Pro ($190ish). Nothing else will have to change
As for the rest of the build, there are some other changes.
HDD: Don't touch WD right now. The Blue series is an old, slower model. Instead, check out the Samsung Spinoint F3 1 TB. It's an excellent deal, as it's one of the fastest drives out there right now, yet I've seen it as low as $55. You could get the 500 GB version, but at $55, it's not that great of a deal.
PSU: Not a bad choice, but there's better values out there. I'd look at the XFX 650W. It's cheaper, but more efficient and modular. It's a steal right now.
RAM: Again, not bad choices, but you can do better. At 4 GB, there is the Corsair XMS3 1600 mhz CAS Latency 9 sticks for $40 after rebate. I'd also watch Kingston's 2x2 GB DDR3 kits, as last week they had some for $25 after rebate. At 8 GB, people tend to gravitate to the Sniper series more, as the sticks aren't as tall, which may or may not interfere with an aftermarket cooler. Since you're just gaming, I'd get the 2x2 GB and save money. Nothing you've listed is really using a lot of RAM.
GPU: I like the HD 5770, but if the choice is between it or the 6850, I'd go with the 6850. If you switch to Intel, I'd look at the GTX 460 1 GB as well.
Mobo: If you stick to AMD (and you shouldn't), I wouldn't get the Evo board any more. It's a good one, but it's older. Instead, check out the ASRock 870 Extreme3. It's a bit more expensive, but that extra money buys you a lot more future proofing. You get USB 3/SATA III support and a more current chipset.
EDIT: batuchka's AMD build is pretty good. Swap in my RAM and optical (cheaper) and keep your case (the Gamma only comes with a single fan, and with shipping it's $50, while the 300 Illusion has 4 fans for $55) and it's about the best you can do for under $700. Of course, you can take that extra $100 (and a little extra) in the budget and get a Sandy Bridge build an it'd be an even better build.