I installed the Mac OS X 10.3.5 update on my home iMac 800 MHz G4 the other day, and now the machine refuses to stay up. I don't even run any weird 3rd party apps that sometimes cause this sort of thing. I boot the machine, it stays up for about 2-3 minutes, then it either freezes or the system just panics and tells me that I need to reboot.
My advice? Stay away from 10.3.5, at least until they can fix this. Apple, you're supposed to be better than this.
Update: After some more investigation into the problem, and after trying some of the usual fixes and workarounds without success, I discovered by running some hardware diagnostics that I appear to have some sort of video RAM problem with my iMac that isn't uncommon. There's even a Yahoo group devoted to discussing it. Very sad. If it's this common, then Apple should do a recall and fix it, just as they're doing for certain iBooks that appear to have similar problems.