The combo Likenew linked is great, all but the psu. I just feel better with something a little more reliable.
There's also an Athlon II X2 combo
, then just add psu, case, dvd and anything else you need (floppy? keyboard? mouse? aftermarket cooling?). The Ahtlon II's are a mixed bag, not as strong as the Phenom II's, but settle in around the E7XXX series from Intel (while costing less).
Just remember in games, it's all about cpu speed (the higher the better) and your video card (I'd suggest an ATi Radeon 4870 or 4850).