I don't mean to be a smartass, but logically, if you have a graphics design degree, you'd kind of be familiar with the needs you have for your creations. Again, that's not meant in a negative way, I'm just saying that you're the best judge to decide what you neeed. I'm sure many PCs would be OK for your work. Personally, though, I think working on a Mac is far less painful, the OS environment is more pleasurable--I love the Aqua GUI--the explorer shell on Windows looks tired and worn-out to me. Also, there are less problems with the OS crashing on you while you're working on something very important--I've had my iBook for a couple of weeks now and I never shut it down and it hasn't crashed once. I'm sure some problems WILL arise, but that's to be expected in the computer world.
I'm guessing that you'd be ok with the iMac, perhaps somewhere in the upper-middle configuration. Again, it all depends on what you're doing, from project to project.
Fiddle around with some machines, if you have access to an Apple store. See how they behave. I think one can tell the difference between a sluggish and a fast machine just by opening some simple apps.
does it need a good procesor? memory? graphics card?
Of course! How much of all that do you need? I'd say as much as you can afford. Another factor is, How important is your work to you? If you plan on designing things for a living, you'll want some rather decent equipment. Right?