AMD Phenom II X6 1035T @ 2.6GHz, 8GB DDR3 RAM 1333 MHz, 1GB Asus AMD Radeon HD 5750, 3TB Boot Drive, couple other lower capacity hard drives for booting into Ubuntu and Mac OS X respectively.
I can run pretty recent titles at low-medium for the most part. Some others refuse to boot. Anything poorly optimized for lower end hardware is a no-go.
I play a lot of Garry's Mod because I work on development for several servers there, and it runs great as one might expect. Otherwise, I play various fighting games for the most part, which this machine handles decently as well, though its age is definitely showing when I have to turn down settings in some simple 3D fighting games (like DOA 5: Last Round).