Like the others have said, It really depends on your situation.
I'm currently a Computer Science Student, but I have a pentium 4 3gz. Which is by far enough power to use. As for engineering and using cad, then it's possible you would need one, *If your school labs didn't have the computers already. In my major we have a Computer Graphics course, which we are required to do rendering, but since our lab is run on Fedora, we are able to log into several computers to Render the frames in the background.
But as a normal student, I think anything around 2gz is fine, even for playing games. Depends on what you play, but I had a 2gz Labtop from Dell back in 2002, and it was just fine playing games like StarCraft, etc.. As for saying you HAVE to play Crysis on high resolutions, that's not for everyone. Most students aren't *Computing* students, so I think most are fine with anything that they can play a little tetris and Bejeweled and of course Facebook...