I removed my g3-350/512k and put in a cheap g3-400/1mb and noticed a decent boost in performance. I have 750MB RAM (256x 2 128x2) and it runs very nicely.

I don't think the cost associated with the very high end g3/g4 ZIFF is worth it considering what a whole G4-1Ghz machine goes for these days (and they would have 133Mhz bus not 100 or 66 when using a fast g4).