This is a great build if it's only meant for Diablo III and WoW. It'll be able to play Diablo III on high (for sure), if not ultra (assuming there's an ultra). The GTS 250 is still pretty good. I'm not sure about WoW (never played it before), but I'm sure it'll do at least medium settings on all Blizzard games.
I run wow on ultra on my 9800+; he'll be fine with the 250.
Diablo 3 will be the same thing, at least from what I played in my limited time in the beta.