If I were you...
...I would go for the 12 PB. You get the G4 (for Photoshop) and you get the portability. However, if cost is a issue, you might be out of luck. Check out the resellers for the best price, not Apple.
If you can't go with the PB, go for the iBook. I got to play with one the other day and they are just great. I don't see a huge difference compared to the 17 PB in normal computing tasks (i.e., opening folders, snappiness, iApps, writing email, Word, Internet, etc.); however, in PS and video it's a night and day comparison.
The iMac is nice, but in college portability is key (IMO). You could take the 12 PB or iBook anywhere...the coffee shops (lot's of chicks there), the classroom, and the library. It's total freedom. And remember wireless. College campuses most certainly have WiFi these days...
Think total freedom with the PB and iBook...
Hope this helped...
Chad4Mac