Case: Antec 300 ($60)
Motherboard: ASUS M4A785TD-EVO (~$100)
CPU: Athlon II X4 620-640 ($95-$115)
RAM: Most 2 GB DDR3 1600 CL9 kits from Corsair, G. Skill, Kingston, and Crucial are ~$55-$60
GPU: ATi Radeon 5770 or GTX 260 (around 60 fps at highest settings except Dalaran and a few other places) (~$160-$200)
PSU: Corsair 650TX (~$100)
HDD: Seagate or Western Digital 500 GB (~$60)
ODD: ASUS DRW-B1ST (~$22)
Total: ~$650
To get the HIGHEST settings on WoW, you have spend a tad bit more than $650. However, with combo deals on Newegg, you can easily knock off about $20-$30. Good luck!