I am assuming that your price target does not include the monitor, so here goes:
MOTHERBOARD ASUS A8N5X Socket 939 NVIDIA nForce4 85
http://www.newegg.com/Product/Produc...82E16813131569
PROCESSOR AMD Athlon 64 3000+ Venice 800MHz FSB 512KB 125
http://www.newegg.com/Product/Produc...82E16819103606
DDR RAM CORSAIR ValueSelect 512MB 40
http://www.newegg.com/Product/Produc...145-026&depa=0
HDD 80GB Western Digital 7200RPM IDE 8MB WD800JB 55
http://www.newegg.com/app/ViewProduc...144-122&depa=1
FLOPPY NEC $13.50 Black 1.44MB Internal 10
http://www.newegg.com/Product/Produc...152-005&depa=0
CD + DVD WRITER NEC Black 16X DVD & CD 45
http://www.newegg.com/Product/Produc...82E16827152058
VIDEO CARD Radeon X300SE ASUS EAX300SE-X-TD-128 Radeon X300SE 128MB 55
http://www.newegg.com/Product/Produc...82E16814121538
CASE Rosewill R203A Black Steel ATX Mid Tower 50
http://www.newegg.com/Product/Produc...82E16811147020
POWER SUPPLY (INCLUDED WITH CASE) INCL.
TOTAL 465
I recently put together this list from Newegg. You might want to spring for an extra stick of RAM. IMHO stick with AMD - you'll get a better bang for the buck. I hope this list helps. You won't get a "great" computer for $500, but it would be a good one.