[kretara]

I would go for a 12" iBook. Nicely optioned it would only cost around $1500.
The iBook is missing some features that you would get in the Powerbook, but it is less expensive and is still a great computer.
I think that you will fine the portability to be a deciding factor. Being able to take your computer with you when you are doing research in the library is a massive plus. Not to mention being able to use the computer to take notes in class, work on papers between classes or at the cafe.

I would get between 512 and 1gb of RAM and something like a 60GB HD. Oh yeah, Applecare is a must for either of the laptops or the iMac.