It will run games. But the video card is a bit under-powered. That guy is probably selling his system to buy a better gaming machine.
Here is a new system with warranty for $590
CyberpowerPC Ultra 2064 Phenom II X4 925(2.8GHz) 4GB 1TB HDD ATI Radeon HD 5670 for comparison. http://www.newegg.com/Product/Product.aspx?Item=N82E16883229223
For a gaming system you'd be better off with a triple core CPU and a HD 5770 or better video card if you can find one.