Firing Squad has a break-down of a $500 gaming PC upgrade. Roughly the components are as follows:

Component Suggestion Price Supplier Notes
GPU NVidia GeForce 8800 GT $249.99 Zipzoomfly Alternatives: Radeon X1950 Pro
CPU Athlon 64 X2 4000+ $75.90 Zipzoomfly Newegg has same price for X2 4000+ and X2 4800+, $99.99. Alternative: Intel Core 2 Duo E4300
Motherboard Gigabyte GA-MA69GM-S2H $79.99 Newegg AMD-690 chipset, micro-ATX. Alternative: MSI K9AG Neo2-Digital (ATX).
Memory OCZ PC2-6400 Paltinum Revision 2 Dual Channel $79.99 Newegg $35 mail-in rebate=$44.99

Since this is an upgrade article, the writers have assumed you already have a case, hard drive, optical drive, mouse, keyboard, monitor and OS.