Ignore these PC bigots. Me, I go both ways. PC for fun with (relatively inexpensive) tweaking and gaming, Mac for efficiency when real work needs to be done and I can't afford to be distracted by games and troubleshooting. For you PC bigots, yes they are stable too, when you aren't in the continuous upgrade mode.
Sadly, if you want a CPU upgrade for your G4, though, they are probably right. I am not sure you could find a worthwhile CPU upgrade for a 500 MHz G4, and if you could the price would probably have you looking at one of the dual CPU 887s instead. If you want to look, I suggest starting with the
www.xlr8yourmac.com web site for info. Other World Computing (eshop.macsales.com) sells upgrades.
For the other components, except video card, PC parts work fine. Newegg has supplied many of my Mac upgrades and they have a Mac section now.
The video card upgrades available are more limited. A Radeon Mac will be a relatively inexpensive and significant upgrade to the Rage 128. Radeon 8500 Mac should also be easy to find. I haven't seen retail versions of the newer cards that Apple is now shipping in the new models that work with Quartz, like the Geforce 4 4600. Even if they are available, be sure to check the specs on your Mac's AGP slot. The older G4s only support AGP 1x - and some of the newer cards no longer do.