Are you going to reuse the harddrive, Power supply, cd/dvd, and case? I'm gonna assume your not going to overclock.
If so you could go with something like
Socket 939 motherboard with built in sound and PCI-E ~$60
Socket 939 AMD64 3700+ ~$90
1 gb ddr ram (if your not overclocking it doesn't matter what you go with to much so some G.skill or crucial or kingston would be fine.) $~100
Nvidia 7600gt or ATI x1800gto (I'd say for nvidia go with BFG or EVGA or Leadtek. for ATI I would go with HIS or sapphire.) ~$150
Total would be around $400 depending on exactly what you went with.
If you are reuseing the hdd, ps, cd, and case and want to go with the $500 budget you could probably go to
socket am2 motherboard
am2 amd64 X2 3800+
1gb ddr2 ram
same video cards
You could get a less powerful video card like a 7600gs or x1600pro if you want to save ~$40. WoW doesn't really require that much of a video card.