If you can't buy a G5, get more RAM! Now! 2.5-3 GB.
On my G4 there are four memory slots but they say it will only recongize 1.5 g of ram no matter how much you put in it - howver that was for syst 9. I am not sure if OS X will recognize more than 1.5 g of ram or if this is a hardware problem.
(My G4 was 400 mhz "AGP graphics" with a 1.2 Sonnet upgrade board).