If your gonna build a "beast" be sure to install one harddrive for each instance of wow you plan to run (I use 5 harddrives to 5 box, some people say symlinking is faster so I will actually have to give that a go and see what kind of numbers i get)

A Single GTX 280 will be fine, I dont know that you will get any noticable gains by running 2 and in fact in some tests running 2 may actually reduce your fps

Be sure to throw in two 21+ inch monitors an X-Keys Pro and a UPS and you should be good to go.