When you purchase an Apple system. You are not purchasing a computer, but you are purchasing a complete system.
I don't understand this logic among the mac community. With a Dell, for example, you get a monitor, keyboard, mouse, and computer. With a mac, you get a monitor, keyboard, mouse, and computer (read: same thing). In fact, the Intel Core 2 Duo that goes in the mac is the same one that goes in the Dell - as is the hard drive, ram, etc... From a hardware point of view, the only difference between the two is the nice case that houses the components and the logos. So, for example, why should an outdated Nvidia video card run better merely because it's inside of a mac case? Answer: It shouldn't - and doesn't. Apple has spent so much on marketing and their image, that the logo on their computer commands a price premium.
The only practical difference between the Dell and the mac is the operating system. I will concede that OSX has its advantages over Windows, but that's not to say that it is the superior OS. OSX has its share of faults as well. It all comes down to what works better for you.
As far as graphic design, you can get high quality, comparable software for either platform. The adobe suite works well for both.
To me, it sounds as if you are looking for a reason to justify a mac purchase. If you just need a power computer to do media/graphics work, buy a Dell, or build it yourself. Take the money you save, and buy a new (or used) mac mini. That way, you can try OSX without breaking the bank and still have a nice PC for your graphics/media needs.
And for the record, I'm not a windows fanboy. I'm just a computer fan, and can appreciate the advantages of both platforms. I have a Macbook, a nice PC, and I had a Mac Mini.