I'm not a pro but use FCP 3 at home. Anyway I have a 20" dell wide screen monitor and like that. For a pro maybe more real-estate would be better so perhaps a second screen or larger one would be better. I figure pros are dealing with more overlaying video tracks. If you have access to an apple dealer with FCP installed and on differenct size monitors, perhaps this can give a clue to the layout possibilities.
What ever you do, do NOT buy cheap ram. Macs are picky about the quality of ram so only go with name brand. Also don't buy ram from Apple. They charge way too much. Try one of the third party online web sites familiar with Macs.
The latest versions of FCP take advantage of higher end video cards. This is because of low level OS X 10.4 video and graphic routines that can take advantage of the newer cards. I'm not sure about the newer G5 towers, but upgrading the video card may give you more oomph! It will help in the realtime rendering during editing. Yes, you still have to properly render the final product. I think FCP also takes advantage of multiple processors, so the new quad machine would be sweet.
Go to apple.com and click on the support tab and choose Final Cut Pro under the Applications icon. Also check out the Discussions area for FCP problems and solutions.
A few people have had serious problems with Apples pro apps. Often caused by their own faults. That is one reason why I mentioned memory above. And I don't know how many times I've seen some do an OS upgrade while your working on a project, and crying that they broke something. This is complicated software it is best to wait for others to find possible problems.
Good luck with the change.