If you want to buy ram and install tiger, and your processor doesnt meet the 867mhz requirement, type this into the open firmware
1. dev /cpus/PowerPC/G4@0 Press enter and ok will be displayed,
2. d# 867000000 encode-int " clock-frequency" property
3. boot cd:,\\:tbxi
Push enter after each of the lines, i numbered them to make it a little bit easier.
I'd recommend having at least 1.5-2 gigs in your powermac if you want to run leopard on a slower processor, otherwise leopard will lag.
But if i were you, and my experience with leopard on an unsupported G4, just run Tiger, it will run much better.
But also Note: Make sure that you type that in with all of the space's and capitalization that ive put there, otherwise it wont work.
Good Luck.
Http://crucial.com also has a system scanner that will show you compatible ram upgrades.