i would go with the power mac g4, here's why:
1)Final cut pro is not native just yet and I wouldn't like the idea of editing under rosetta.
2)You need alot of ram to edit and From what I remember you could put more ram into the powermac.
3)The ability to use two identical screens. two screens are very usefull when editing especially when they are identical(speaking from experience it's annoying when you use two unidentical screan when editing)
4)Expandibility and Upgradability, The g4 processor, ram and hard drives could be upgraded. Powerlogix makes a dual processor upgrade for the g4 if your g4 is a single processor. You can't upgrade much on the imac if anything.
5) You'll get a better machine for editing and "prosumer work" (ie. editing animation, 3D) for your money.
I'm not bashing the intel macs, i would just wait until rev c to get one, or a G5 imac would be better i saw in another post realworld benchmarks indicate only a 15% increase of speed rather than the 2x or 4x they are claiming. i would rather just max out my ram rather than buying a rev a intelimac to get the 15% increase.
honestly! there is just way too much hype about these new machines, people will tell you to buy it because initially they look better, but, my best advice for you is to do your research first and explore your options. find out what you need the computer to do and what are your expectations, i'm sure you could find a older mac to do whatever you need. I have a dual 2.0 g5 and do animation, design and video on it, but, i also did all those things on my single processor quicksilver with the same results( granted slower rendering but not that much slower). Newer and faster isn't always the best option. You'll save alot of money that way.