I will take my stab at this…
SAMSUNG 22X DVD±R DVD Burner Black IDE Model SH-S222A – OEM
Item #: N82E16827151175
$25.99
Antec Three Hundred + BP430 Black Steel ATX Mid Tower Computer Case 430W Power Supply – Retail
Item #: N82E16811129065
$89.99
Western Digital Caviar Blue WD3200AAKS 320GB 7200 RPM SATA 3.0Gb/s 3.5″ Internal Hard Drive – OEM
Item #: N82E16822136074
$49.99
SAPPHIRE 100255HDMI Radeon HD 4670 512MB 128-bit GDDR3 PCI Express 2.0 x16 HDCP Ready Video Card – Retail
Item #: N82E16814102820
$64.99
OCZ SLI-Ready Edition 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model OCZ2N800SR4GK – Retail
Item #: N82E16820227269
$45.99 – $10 MIR
AMD Phenom II X3 720 2.8GHz Socket AM3 95W Triple-Core Black Processor Model HDZ720WFGIBOX – Retail
Item #: N82E16819103649
ASUS M4A78 PRO AM3/AM2+/AM2 AMD 780G HDMI ATX AMD Motherboard – Retail
Item #: N82E16813131362
$198.99 Combo
Subtotal: $475.94
Shipping: $10.57
Grand Total: $486.51
Now I know this isn’t high-end and MasterP will find reasons to complain as he always does but that computer will play every game out using at least medium settings and is the best you can do for that price. I would recommend spending an extra ~$70 on top of that though and upgrading to a phenom II x4 920 and a radeon 4850 but still for under $500, a decent gaming computer is within grasp.
Also for MasterP who doesn’t know, the radeon 4670 is basically the same as the 3850, which was a pretty decent card and actually can play games at a budget price.