You didn't say where you're buying from, but right now this is what I'm building for a couple friends who slightly more than casual gamers (rough Canadian prices with pricematching)
AMD X2 555 3.2ghz dual core ($110)
Gigabyte GA-770TA-UD3 motherboard ($90)
4Gb G.Skill DDR3 ($115)
5850 Vid card ($300)
+ 640GB Western Digital hard drive, 23" ASUS LED monitor, Win7 64-bit, etc.
I'd never get a cheap PSU, it's the most important part of your computer. Corsair, Antec Earthwatts or TruePower, PC Power & Cooling. I went with the Antec Earthwatts 500W ($70) inside Coolermaster 690 cases ($75) + a couple extra fans.
The important thing that the motherboard has is ACC, that gives you a very good chance of unlocking the 2 extra cores of the X2 555 processor, effectively turning it into a quad core 3.2 X4 955 very easily
If that doesn't work (the majority of CPUs should unlock, so far I am 100% - 3 for 3 have unlocked, with 3 more shipping this week), you still have a very powerful 3.2 ghz dual core that will overclock easily to 3.8ghz with no voltage increase. The downside I guess to that motherboard is that it only has 1 PCI-E x16 so no crossfire. But that's a non-issue running this card at the resolutions for this monitor for a long, long time. By then they will just build again or just replace the card.
But yeah. That's why I'm doing these X2 555 builds for my buddies, great CPU.
Total system cost ends up around $1200.