If you go the laptop route -- I would strongly recommend a 7200 rpm drive.
There's only one out there in the 2.5" size though. It's the Hitachi Travel Star 60 GB [you'll want to check the model #]. There are a number of models from several brands that can be configured with them. I have one in my Dell Pentium M (Centrino) machine and it's quiet and fast. You can do a replacement yourself in some circumstances. Do NOT get a machine with a 4200 rpm drive for serious audio or video work unless you're planning to haul an outboard drive around with you. (And even then, c'mon!)
If you get a PC, you'll have to go through the headache of a platform switch. Depending on your circumstances, it could be a breeze or a big pain in the A. There are a lot of good deals and a number of directions you can go, depending on your needs. I like my machine more each day -- and it's almost a year and a half old and was refurbished when I bought it heh -- but I'm honestly afraid it's setting such a high bar I'll be disappointed when the time finally comes to get a new laptop.
OTOH, I plan to have built a new desktop by then -- almost certainly around some kind of Pentium M, I'm thinking now, since it's one of the coolest running chips around -- and after 19 years of 24/7 computers, when I got this laptop and finally turned off my old desktop it was like, wow, what's that noise? Silence? Dang! So, I'm trying to set things up so that my next laptop can be fairly modest. (Then again, aside from the fast drive, I was thinking this one was kind of modest... I never really thought I'd be able to do so much with it, particularly with audio. All gravy.)
If you stick with Mac, you'll want to stick to Firewire outboard drives if at all possible, because of poor USB2 performance on Macs. (You can check out various Mac models and HD and CPU performance
here, among other places. Take special note in the drive performance section of issues with the FW, PCI, and drive controllers.