I have a Mac Mini (1.42Ghz, 512MB memory) but I don't really use it for the high-demand applications.
A quick'n'dirty test using Capture 1 Pro 3.6 on the Mac mini and a Thinkpad T40p (1.6Ghz Pentium M, 1GB memory) showed that the Mac mini is around 30-40% slower in processing the same amount of RAW files. Needless to say, my desktop PC (3Ghz Pentium 4) stomps all over the Mac mini in graphics processing.
What do I use the mini for? Right now, accessing the web, doing email, doing some small projects for the kids - it has become the de-facto computer for day-to-day use. I only fire up the PC when I need to do the graphics stuff.