For WoW a 9800GTX IS overkill. I have 2x9600gt's which barely hedges ahead of a 9800GTX and when I do play WoW, which isn't often, we're talking HELLA frames per second.
So a 9800GTX would definetly be overkill.
Go with a Q6600 and a good aftermarket cooler. If you're going to overclock your Q6600 then you'd want a Tuniq Tower, but if you're not, any good aftermarket air cooler would do just fine.