GIMP can't touch CS5 in terms of capability, or performance. And before you say it runs faster it your pentium4, remember that PERFORMING isnt the same as GOING FAST. You have to be able to get things done, and CS5 can do more things.

Also: the newest Mac stuff runs on the same hardware PCs do, including pentium processors, albeit a generation behind the enthusiast offerings. I don't think you can even buy a mac with a GTX480, but i havent browsed in a while. The OS is more stable only because the software for is developed by the same company that makes the computer, ensuring compatibility. If Adobe made computers, everyone would buy them for premier, CS5, elements and flash would always run great. But since the best software is going to be made for the largest market, its easier for you (and cheaper) to build (or buy) a PC.

And besides! You can always go buy a copy of OSX and dualboot it, although making a hackintosh isn't quite as easy as dualbooting some flavor of linux