Definitely get one of the newer Core Duo machines. I'm still using my PowerBook G4 - for over 3 years now, and although it still works fine, it's slow as a dog compared to the newer ones. I have a MacBook 13" i use at work, and even though the smaller screen sux, you can run dual monitors no problem, and the clarity of the screen is awesome!
Only "problems" I'm having with my PowerBook is that opening Adobe programs, and working on large photoshop/illustrator/inDesign documents...it takes longer than it should to open/modify these documents at times--and this is with 2 gigs of ram.
I still love the machine, and depending on what they come out with in October/November, I might pawn this machine off to my folks, and pick up one of the last machines prior to the switch over.
I'm not a gamer, but I'd say talk to some mac users that are gamers...not sure if you could just run some of your games off of Boot Camp--that way you wouldn't hve to drop another 1500 on a windows machine...just plunk that down for a mac...and if you want to get really crazy and don't need to be mobile...get the 24" iMac....DEAD SEXY!
Marc