I have just purchased a new 17" iMac G4 and upgraded to 512Mb of Ram.
With 256 it worked fine although I got a few warning signs when creating large DVD projects (whilst also having other apps running).
With 512 I have yet to see any warnings at all no matter how hard I push the system (the heaviest so far being creating a DVD from a 4Gb DV file whilst also importing from audio CD's to Mp3 and browsing the web).
Possibly, when I start editing video in anger, I will find the need for more Ram but it hasn't happened yet.
I installed a bog-standard 256Mb SO-DIMM card (266Mhz DDR, cost 34 UKP) and it went without a hitch, fo another 120 UKP I could have gone to a 512Mb SO-DIMM card but I reason that the price will drop considerably in the next 12 months as laptop users start to buy these cards in earnest so it is better to wait a while until the price drops.
After the 1 year warranty is up I may divest this dome-bot of it's casing and attempt an internal upgrade (512 DDR Dimm is really cheap!) and if I do I'll report the outcome here.
btw, as far as processor speed goes a 1Ghz G4 sems to be equivelent to a 2.4 Ghz P4 although you also have to factor in the graphics engine which in the iMac case is far more integrated with the OS hence giving seamless operation. A 3Ghz P4 PC will do certain jobs faster(like converting DV to Mp4) but overall the PPC G4/4MX video system performs better on the desktop.
Amen-Moses