Well this is what I could come up with for a sub $500 PC (without monitor)
These parts are from NewEgg
NEC 16X Double Layer DVD±RW Drive, Silver, Model ND-3500A S W/SW, OEM with software $63.99
Item# N82E16827152030
Cooler Master XDream II 80mm CPU Cooler for Socket A/370, Model "HAC-L82" -RETAIL $12.49
Item# N82E16835103146
Western Digital Special Edition 80GB 7200RPM IDE Hard Drive, Model WD800JB, OEM Drive Only $60.00
Item# N82E16822144122
Logitech Media Keyboard PS/2 104 keys -RETAIL $21.99
Item# N82E16823126160
Kingmax 184 Pin 512MB DDR PC-3200 - OEM $64.50
Item# N82E16820156006
ALBATRON "KX18DS Pro" NVIDIA nForce2 Ultra 400 Chipset Motherboard For AMD Socket A CPU - Retail $52.00
Item# N82E16813170122
Microsoft Optical Wheel Mouse BLACK PS/2 /USB Version 1.1a -OEM $14.99
Item# N82E16826105143
AMD Athlon XP 2500+ "Barton", 55W 333 FSB, 512K Cache Processor - OEM $86.00
Item# N82E16819103506
ROSEWILL ATI RADEON 9600 Video Card, 128MB DDR, DVI/TV-Out, 8X AGP, Model "RW9600-128VD" 64Bit -RETAIL $66.00
Item# N82E16814164023
Product total: $441.96
Shipping & Handling: $ 28.37
Plus this case from TechSunny for $20 + shipping w/ 400W PS
Case Link