I have actually run OS X 10.5.something on my built-from-part desktop PC... there's a certain set of hardware that works really well with OSX... for example, it picked up my desktop's Marvell Yukon NIC easily, but failed completely on my HTPC which has a Realtek NIC (I googled around a bit, but just couldn't get it to work).
I only used to use it to link up the firewire port for my old Comcast Scientific-Atlanta HD-DVR box (couldn't get the right STB drivers under Windows XP... and the OSX stb firewire package just worked). But I did play around with it for a bit... was quite nice to see an environment where Quicktime actually worked smoothly and didn't die on 1080p video...
As someone pointed out already, check out the osx86 project forums to get an idea of "ideal hardware" for running OSx86.
EDIT: found a picture on my laptop of that set-up:
(it's a split-screen between OSx86 running on my PC and my HD-DVR box)