Any system able of run Gnome 2, KDE 3.2, Windows 2000, Mac OS X and later versions should be capable to run GIMP. GIMP's biggest appetite is for memory and how much you will require will depend on the how many images you are working with, their resolution, size, and the number of layers you use and so on. As a common rule, more is better, but 128MB should be enough to be getting started with.