Well, as a student, here's my take... Sounds like you are a PC user atm, as am I. But you are looking to 'switch' presumably for the improved user experience, and not the gaming available!
So I would point you towards grabbing a high end 15" powerbook for now. (well, wait untill closer till the school year starts, if you look around the forum, you will see it is my opinion, as well as many others, that new PB models are on the way).
As mentioned earlier, the 15" PBs will run just about any of the games available right now with any issues. If there is a decent bump in PB specs this summer (slightly faster CPUs, more vram) then it'll just be gravy.
For general computer usage, you can't go wrong with a new PB, especially the 1.25ghz w/ the superdrive. It'll probably get you all the way thru med school for non-gaming usage!
Anways, that'll be significantly cheaper than getting a powermac, and you probably won't miss the extra power right away. By the time you start to see games come out that require more 'juice' the next generation of consoles will be hitting the shelves. From all accounts the XBox2 and PS3 are going to pretty much low away PC based gaming anyways, and more and more games are going to be first run on consoles. Grab a nice HDTV then too, you'll still be spending less than you would have on a powermac. Your DVDs will look oh so nice then as well!
The point is that your PB will still be a perfectly viable computer for general use and light, on the go gaming, and you will still get all the newest video games. Plus when the next greatest video game systems come out, you will only have to invest another $300 or so, which about the cost of the latest and greatest video card for a computer.
As a current PC gamer, and someone looking at making a mac switch shortly, this is honestly where I see myself in about 12-18 months. I think that's the way the world is moving...
Hope all that helps!
Rob
PS I was looking at the ADC program myself. The discount on a PowerBook is about US$200-300 over and above the educational pricing depending on what you buy, and the membership is US$100, so the net savigns of a $100-200 is pretty decent, but it is a ONCE in a lifetime deal (at least thats how I understand it, correct me if I am wrong), so you may want to save it for later if money isn't an issue now and it may become one at a later dae (ie, when Med school bills start to pile up and you are thinking about upgrading). Jus a thought. 🙂