That's gonna work real well for "graphic design school", isn't it? Wow!
gimp does NOT equal Adobe Creative Suite, no matter how hard you try. And since creative suite is one of the things you absolutely critically rely on for any kind of design work, especially anything in regards to print, you'd be pretty much fucked on a linux OS.
If I were you, I'd wait till the last possible second to buy something. After WWDC in August. Rumour has it that WWDC will see the introduction of two things: a MacBook Pro with Intel's Core2 Duo processor, and a PowerMac replacement, also with an Intel chip. Everyone's expecting some sort of hype on OS X 10.5, but whether that be just a big show on what it's gonna do, a prerelease development version, or even a full release, no one knows except that snotty black turtleneck wearing metrosexual (AKA Steve Jobs).
One thing though, neither Creative Suite nor MS Office (which you'll be using a lot in college, trust me) have gone to Universal Binary yet, meaning that you have to run them in emulation until they do. And that means a performance hit. Not a big deal for MS Word, but a big deal for Photoshop. And if you need shit that ONLY runs on Windows, you can now boot Windows Xp (and the Vista Beta) on all Mactels, so you don't have to settle for poorly designed and poorly integrated hardware.