There is a big difference between the Dell Inspiron 8600 Spanky Deluxe mentioned and the 15" PowerBook - the weight. The 8600 weighs 6.9 pounds with minimum configuration and the 15" PowerBook weighs 5.6 pounds. That is a very noticable difference.
Last fall, I had an Inspiron 8500 and a 15" PB. To carry the 8500 without burning my shoulders I had to buy a huge Targus backpack. The 15" PB fit perfectly in my messenger sling and I had no trouble with it. The 15" PB is still a fifteen-inch laptop, but the less weight is a huge factor.
crazydreaming, if you want to get a nice laptop to last for several years, I would recommend the 15" PB. It is certainly portable despite its size and you should never have any trouble with a screen that is cramped. The 12" iBooks or PBs are very nice, but sometimes it is nice to have a larger screen to work with. The composition of Macs at my university is heavily bent toward the 15" PB because it is a great all-around machine.
With that said, noticing from my signature, I have a 12" PB. But that's solely because I had a 15" PB for a while and wanted something as portable as possible. You could say that I'm the uber-mobile user and I don't mind the smaller screen real estate. I'm on the extreme when it comes to portability needs, so don't be under the impression that I'm urging you towards a machine you'll regret. The 15" PB, for the needs you listed, will keep you happy straight through university.