I've played wow extensively on a 1.67ghz G4 powerbook with the 128mb ati card and a 2.0 (non SR) MB that is listed in my sig. Both machines run WoW just fine at decent settings. While the MB's integrated chip was not as good as the older PB, wow is still perfectly playable on an MB.
Don't break the bank if all you are going to do is play wow. Use the leftover cash to up your hd, buy a good external, and add 2 gigs of third party ram.