You'll probably end up using most programs that aren't autocad (depending on that school), rather more programs like adobe shit (illustrator, photoshop, indesign). Regardless, you don't need to shell out a ton for a laptop, since most new models have sufficient hardware to run it. But memory is your friend, so load up, whatever brand the computer may be.
In the end, I'd avoid macs. While they are good computers, there are some computability issues (namely with something like rhino) that really bites ya in the ass. Not to mention they cost a shitton, and you'll probably end up buying quite a few programs before you're done. Something to keep in mind.