Well, Gmail can forward email as POP, and Apple's Mail.app is really nice n' functional n' whatnot. Unfortunately I'm not familiar with a way to import Outlook email into Mail (Mail's own help says you can't). Still, I guess you could forward it all into Gmail and then download it later into Mail.
Incidentally, if you do pop forward your Gmail account, the mail will be archived on the server end. It'll even take a copy of any emails you send through Mail.app and keep a copy in Gmail, so you always have two mirrored versions of your email offline and online.
Anyways, It's an awesome little operating system and I thoroughly recommend it. Having said that, not everyone seems to 'gel' with the system, and I'd suggest you have a good long go with it for a few weeks before buying - say, borrowing an old machine off a friend if possible.
Also, as soon as you get a mac, install Quicksilver and play around with the tutorials provided at places like 43folders.com. It's an incredible application that has completely changed the way I use a computer to such an extent that I can't use a computer that doesn't have it installed. any more. It's simply revolutionary.