i find this post to be incredibly odd personally. I don't know what kind of Powerbook you were using but i am using a 17in 1.5ghz g4 with only 512 ram and i edit perfectly fine. it is extremely rare for me to have any kinds of problems with it. the only time i will start dropping frames is when i run FCP, DVDSP, LiveType, Opera, Aim, Quicktime, Mail, iPhoto, and Photoshop (although i don't recomend running WoW while doing all that too
). the only thing i do see a need for is more ram because i would like to be able to run all those programs on a regular basis together with out having to worry about dropping frames.
I have also used a G5 iMac for editing and i find it to do it fine. it has more power then my Powerbook so it easily does all of it smoother. I personally would not buy a desktop with less then 1.5 gigs of ram. I would be personally fine with a iMac with 1.5 gigs of ram if that was the best i could do with my alloted money. just don't forget that you also have to put in a couple hundred (or several hundred) in for the monitor for a PM dual G5.
Also i would recomend skipping FC express. its a waste of money and is just a stripped down version of pro. I personally found FCP incredibly easy to learn and straight forward. I picked it up with little help in about 1 month and became good with it in about 3.