The newer video cards offer a pretty significant boost in performance. Going from a pair of Radeon 4870s to a pair of Radeon 5850s was the difference between running WoW with 2x AA and most options at half to running with 8x AA and all options at maximum (on a single instance of WOW on one monitor). The worst I get in Dalaran is 16-20 FPS if there is a huge crowd on the screen, otherwise it's 35-60 FPS everywhere. I can only imagine what it's like with the more powerful cards (5870, 5970, the NVIDIA 470/480).

I haven't tried my multibox setup yet, but it all ran fine on the 4870s and I figure I'll be just as happy with the 5850s. Maybe I'll try this weekend.