[ar1550]

I'm very new to the Mac world (I'm getting my first as soon as Apple ships it), so someone correct me if I'm wrong.
The G5 runs at a much higher FSB than the G4, and uses significantly faster RAM. The current G4 Powerbooks use PC2700 (*CORRECTED*), which is pretty much the slowest DDR RAM you can get; I don't know what the G5 uses. If you buy ram that is much faster than what the PB G4 can take advantage of, it may be useable later, but obviously no guarantees. Most ram will run just fine at slower speeds than it is rated.
Going with dirt-cheap ram is rarely a good deal, as it can be unstable. I've head on various forums that Apple computer are even more sensitive in this way than PCs, so it's likely any no-name ram you go with will cause more headaches than the savings are worth.