If you use that machine for a LOT of video editing, you may find that an additional 250GB drive would help for storing all your clips and sounds and what-not.
I would suggest, at a minimum, 1GB of RAM for what you want to do. If you can afford to go higher than that, then by all means do so, as more RAM will help with rendering and overall performance.
If you've been working with Final Cut Pro for a while and know your way around the keyboard commands pretty good, you may be interested in the Contour Shuttle Pro input thing (
http://www.contourdesign.com/shuttlepro/). It's great for scrubbing through clips and general FCP use. The website doesn't list G5 compatibility, but then again, they don't say that it won't work with the G5 -- I don't see why it wouldn't, but you may want to give them a call just to be sure.
The graphics card that comes installed with the G5 is great, but the upgrade to the 9800 is even better. For an extra $300, that decision would lie with you and how much you'd like to spend on that.
I hope that helps a little!
