If your the "creative" type DEFINITLY GET A MAC (and make sure you get an Intel CPU).
I'd get a desktop for the most/easyist upgradability.
I just got an iBook and I ONLY use OS X for everything except audio processing and recording.
It is cheaper to build your own PC though. And it's pretty easy too.
Agree on all this, depending on your budget of course.
If you're rich, get an Intel Mac. But right now it would still cost you more than they're worth (and you'll lose out on the price drop when the next upgrades are released)
If you're on a tight budget, a PowerMac G4 would still cut it.
If you're a hardware techie on a reasonable budget, assemble your own PC.
And before you get a laptop/iMac or similar, consider expandability.
My uncle picked up an iMac, decided he wanted to import analogue video and realized there were no PCI slots! Yes, he sourced for an external breakout box, but the functions are limited and cost even more...
/Jan