The 260 is basically equal to the 4870, keep this in mind for what i'm about to tell you.
I had an E8400 & ATI 4870 running WOW at full settings (except for the shaders, i had those @ half) all on a 22 inch monitor running 1650 x 1050. I tried to crossfire in another 4870 and i actually lost FPS. In otherwords, SLI or crossfire & SLI are not supported in WOW, so one card is your only option.