AMD Phenom II X4 940 3.0ghz Quad Core
Mainboard that can run DDR3 (AM2 Socket), Onboard ATI graphics, ATI sb 800 chipset. Gigabyte makes a nice one.
8GB DDR3 800 mhz +
ATI Radeon HD 5 or 6xxx series gpu.
You should be able to get all that for under $500, then get yourself either a nice 22"+ Wide LCD or a blur ray player.