I run MacOS X on a 233 MHz Powerbook G3 (Wallstreet) with 320 mb ram. It's a bit sluggish, but I guess a faster processor would help with that.
I run XDarwin with OroborOSX on it, but it has a danish keyboard, and the default keyboard map in XFree doesn't fit it at all.
So, I've done a quick hack, on a Powerbook .Xmodmap I found somewhere on the net. You can download it here.
It might also be useful if you're running LinuxPPC.