you want something with a video card that can handle games on mid settings. so something with these specs.
Single core AMD Athlon 3200+, and just Overclock them urself
a good oc'ble mobo, like MSI or DFi
a video card should be like a 6600GT or 6800GT
a monitor with less than 12ms response time. or if ur going with CRT monitor than something with a good refresh rate
any case that has good airflow so that you can OC it, u might wanna buy an Aftermarket heatsinkfan
any hard drive is fine, 250GB? western digital caviar, its up to u for the size
1x512MB Dual Channel ram (DDR, not DDR2)
that should be good.
u are building urself right? that would save u A LOTTTTTTT of money than getting it prebuilt.