I replaced a NVIDIA 8800GTS and NVIDIA 7800GTX cards with ATI Radeon 4870s and they work great. One system runs a single WoW instance at 1920x1200, the other runs four WoW instances at 800 x 600 (1600 x 1200 total).

I went with the 4870 because the performance looked very good and the price was much lower than NVIDIA's at the time ($290 for the radeons, like $400 and up for the NVIDIA cards). I think that either card you get will do the job, so shop by preference or price.