For World of Warcraft that setup should be fine i would think. When i played wow i ran a 5770 with 3 monitors with a good 30 fps. that 660ti should be more than enough. id try it first before getting another video card. WoW is ALOT less taxing than many other games. and its more CPU bound than anything.
Triple monitor setups use more Vram than normal, which is why 2 videocards are sometimes "better" but you do run into more problems, and some games dont use crossfire effectively, if at all.
660ti has 2gb if i recall correctly, for wow, thats fine.
But if this is a build you are considering on buying, then i would drop down to 8gb of ram and put money for a higher GPU if possible anyways. 8GB is more than enough for gaming.