Will you be using OS X exclusively, or will you be trying Boot Camp? If the latter, make sure it's only for one or two apps you can't get OS X equivalents for. You don't want to reboot constantly if you can avoid it, and you should remember that Boot Camp is still a public beta (albeit a pretty stable one).
In terms of performance, WOW will play properly in either OS X or Windows; it's faster in Windows, but from what I've seen recently it's more a matter of Blizzard needing to implement further optimization than anything OS X is doing. I'd probably stay in OS X just to avoid hassles. Regardless of platform, you'll at least be able to run multiple apps at once, since the Core Duo will take care of background tasks.