abit LP35 $170 intel q6600 $200 adata 2 gig ddr2 800 $45 sapphire hd3870 $150 cooler master centurion d $50 corsair hx520/620 $150/200 (depending on if u plan to crossfire if u got extra bucks) zalman 92mm $35 if you want to overclock, an arctic freezer if u dont need to for $20
about $780-800 without keyboard, mouse, monitor, speakers/headset, harddrive, or media drives and will play unreal engine games with 70fps (ut3, bioshock, vegas, gears of war) crysis on high about 25fps, and handle newer games that use multicore like a pro.
You'll probably want to go for a case, motherboard, cpu, some RAM, a video card, a sound card, maybe a wifi card, keyboard, mouse, monitor, a hard drive, and perhaps some speakers and a headset.