My OLD (very old...) 9500GT could max it out at 1920x1200, and get 30+ FPS.
The GT 240 would be the best, and not so expensive.
XFX has a nice card!
For the most part, it depends more on the CPU and internet conection
actually WoW is a dual core game, four cores won't offer any increase and with shadows on it becomes rather GPU dependent. It's no longer the 2004 graphic engine.
This is true and is able to take advantage of sli and crossfire. It can use more than two cores as well after a recent patch but didn't impact performance at all. The recent patches really beefed up its foot print on the system.