Luckily upgrading those G4s will be pretty easy. By the time the G3s rolled around, Apple had switched over from expensive EDO RAM to plain old PC-100/133, and they've been using IDE hard drives since the switch to PowerPC around 1992/1993. I think those may be some of the very last Macs that can natively boot into OS 9, if you want to give that a try. (I like that OS a lot, but maybe I'm just nostalgic.) You
might be able to run Leopard (10.5) if any of them are 867mhz or faster (or if you use a program like XPostFacto to trick the bootloader).
Max out the RAM, throw in a couple good size hard drives, and dual boot OS 9.2.2 and Tiger.

Even if you don't use OS 9 all the time, install it anyway so you can load Classic in OS X and run pretty much every Mac program made since 1984. (Then go Inception on it and load Mini vMac inside Classic...)