I would start with what you want to do with your machine: simple email, internet and word processing or Graphics and design. That's always a good place to start. Then base your decision on what needs to be on that machine to accomplish it and price it accordingly. I would say check at your school to see if there are any packages they provide, but based on what I saw you won't save much.
I think the machines you are looking at all have AGP graphics, which is good since you'll need it for Jaguar and beyond.
But sometimes getting more than you need now is better than getting something lower that you'll need to fix soon. You can wait until the 970s are released and see what that does for the prices. If you don't have a lot of money to spare (or the tech expertise) maybe you should be looking at a Apple solution. These other solutions may require to install other things to get it to the point to do your work effienciently. I bought my first computer retail when I was in college (an Amiga 2000--I loved that old machine

). And it worked for my well for my school work; if anything went wrong there was an Amiga dealership in town I could take it to--no fuss no muss). Maybe you should do the same thing, so it will be one less thing to worry about...
Terrance