I would say get an asus mobo (I have the P5E, it's kinda expensive cuz it came out a few months ago, you can get a cheaper one that's probably just as good), evga superclocked 8800gt, intel core 2 duo e6850 (i guess get quad if you want, kinda useless as of now and expensive), a good heatsink for your cpu (zalman 9500 is great), corsair ram (2gb is plenty).
This is pretty much my build right now, just built it a few weeks ago. I can run crysis on high 1680x1050 with an average 30fps, bioshock 1680x1050 max everything with constant 60fps, gears of war max 1680x1050 constant 60fps with lots of stuttering (which isn't uncommon, they really need to patch this).
edit:
oh, use newegg.com for the stuff. They have some great deals and the 2gb corsair ram is like $47 right now after MIR.