If you want to keep your old G4 400 MHZ running, I'd first suggest a visit to the Sonnet web site
, and at least think about a processor upgrade (a 1.0 GHz Encore/ST will run about $200.00).
It sounds like you have plenty of RAM, so that won't be an issue.
Before upgrading to OS X, make sure that your OS 9 is updated to 9.2.2, and all the other updates via Software Update have been done (especially any Firmware Updates)
I currently run a similar configuration, and I have very few complaints...