The Mac OS X is much much superior to windows or any PC based OS. I've been using photoshop for a long time now, and I know what the difference is when you get to work on a mac. I have a PC 2 Ghz, 640 MB RAM and a good graphics card, but the iMac G4 beat the PC fair and square when it came to image processing and the likes of it. Its too good. I don't own a Mac, I used it at a friends place. Believe me, there's a big difference...
I must admit that though the Mac is so good, it doesn't really justify the price which is halved in the PC. So, the conclusion is, if you have budget restraints, go in for the PC, otherwise, for graphics and image editing, the iMac is the way.