I built the following PC running Windows 7 64 bit for approx £400 ($593, but prices most likely cheaper in US than the UK) a few months ago. I used ebuyer.
This setup runs WoW on Ultra video settings, in 25 man, with all addons running (of which I have many) at between 50 and 60fps, though to be honest I never notice any deteoriation.
For the elite gamers out there I do not claim this is the "best" setup, just what I have experience of myself. I have missed out RAM as I purchased from a friend, I have 4GB but I can't recall the exact specs right now. I also had monitor, keyboard, mouse etc. so did not need to purchase them.
If you're a student check out Microsoft's offers on Windows 7 upgrades etc.
1 x ASUS P5QL PRO iP43 Socket 775 8 channel audio ATX Motherboard £51.68
1 x Sony AD-5240S 24x DVD±RW & DL SATA - OEM Black £13.06
1 x Sapphire HD 5770 1GB GDDR5 Dual DVI HDMI DisplayPort HDCP PCI-E Graphics Card £113.32
1 x Arctic Power 600W PSU - With PCI-E, 3x SATA, 20+4, ATX12V, 8pin +12V Connectors - Retail Boxed £30.42
1 x Piano Black Mid Tower Case 12cm Blue LED Front Fan and space for 22cm Side Fan - No PSU £12.50
1 x Seagate 750GB Hard Drive SATAII 7200rpm 32MB Cache - OEM £46.41
1 x Intel Core 2 Duo E7600 3.06GHz Socket 775 1066Mhz 3MB Cache Retail Box Processor £89.50