I agree with the beautiful Blue Velvet, it comes down to whether you want a laptop or a desktop. Obviously the top of the range Mac Pro will be the best in terms of performance, but not necessarily in terms of value or ease-of-use (portability). You'll probably want something with a dedicated graphics card, a decent hard drive (size and speed) and a good screen. Back to the basics though; laptop or desktop?
For graphic design work, you generally need a big screen and a relatively powerful machine. That means the PowerMac with a nice external monitor, or a 24" iMac. On the other hand, if your cash strapped, plenty of people get by with less expensive but less powerful systems.
That's just some general advice though. Tell us more about your plans for the machine and we can get more specific.