9800M GTX in SLI is faster, but WoW isn't really optimised for SLI (not saying it won't work, you just won't get much of an improvement). GTX 260 would be better if you are only concerned with WoW fps
(GTX 260M is just a higher clocked 9800M GTX)

*edit*
Expect the 260M to perform similar to your Desktop 9800gtx. (if I read your signature correctly hehe)