I have a centrino power laptop which I have used linux on and off on for th last 6 months. It has a 14.1 screen and the 1.3ghz pentium M processor and while it has a relatively small battery, I can get 4-5 hours under linux without to much of a stretch. However, it did take me quite a while to get power management ect to run the way i wanted it to (although the newer kernels are doing pretty well).
Also with the integrated intel prowireless 802.11 adapter, it works for me under Knoppix. One of the problems with laptops though is that they are pretty much all differant. Id try googling for the model you are looking and linux and see if anyone else has done it allready. This would be a good palce to start: http://tuxmobil.org/dell.html
(the parent site, http://www.tuxmobile.org
) helped me out alot when i was looking for reviews of the laptop i bought (ibm thinkpad r40).
As far as getting online, I would be suprised if you could only get online wiht a windows system, but you might have trouble getting help. You might want to try searching the school's website for a linux user's group or searching through student personnal pages and see if anyone mentions running linux under the college's network.