WoW doesn't have very strict requirements, I can run it on my P4 win2k machine (see sig) that only has an Intel IGP and 512mb RAM atm.
you could build a PC to run WoW for probably about $250 or so (do a search for WoW) someone has a thread about wanting to play and someone listed them a build from newegg which ran $250 or $300.
if you were to get that one off newegg, you would need a better graphics card, that one only has an IGP which will run WoW, but not at full graphics which is the whole point of buying a new PC to run mainly that game. also, if you buy this one, I would downgrade to windows XP rather than vista.