I find a brick works wonders.
The upgrades are rather pricy, you'd be better off saving your pennies and buying a new PM G5.
As a stop gap, buy 512MB DDR400 sticks, so that they will work to the full potential in any new powermac.
note: they will only work @ 333MHz in the G4 power mac. If your PM uses SD RAM, I wouldn't bother.