I have a G4 400 Mhz Sawtooth and I upgraded it with a 7447 G4 2Ghz processor from Gigadesigns, spending 400$. I also have 512 MB of Ram which will be soon upgraded too. Now my mac is really really fast, as I have to use it with some video-editing apps.
Just yesterday I found some good informations (...) about OS X x86. There are people who have installed it on their pc, and it's working too!!
Just search and you'll find out more. ;-)