I use a 6800nu default clocked in WoW and all other modern games with no problems. WoW doesn't put near the amount of stress on the system that doom3, BF2, or F.E.A.R. does. WoW is actually playable with my geforce4 video card
If you're thinking of upgrading from a 6600gt to a 6800nu, it's not worth it.. It will only be about 5% faster in directx games (unless you unlock and overclock the 6800). Instead, get a good directx card like the X800XL or unlocked x800gto2 - both will stomp a 6800nu in WoW.