get a quad core cpu (6600 or better)
4-8GB ram
gtx260 or better
720 watt psu
vista64 (home premium for 4GB Ultimate for 8GB - get the OEM version)

the above will run 5 copies of wow at over 40+ fps per instance

the choice is yours, pick components from your favorite manufacturer, you should be able to come in under your price requirements just by shopping around