never played WoW before, but I imagine that a GeForce 6X00 series card and up or an ATi X1000 and up would do you very nicely 
both avalible in AGP and PCI-e
if you only have PCI… I strongly suggest getting a new mobo
edit: most of the cards in that range can be had anywhere from 40.00 on up to the 300.00+ range