EIther an erase and install OR (perhaps more preferably) install Tiger on a completely separate drive in your G5. This way, you will keep a known, solid and stable system until you get everything installed correctly and tested with Tiger. Panther will still be solid and stable on drive A, and tiger will be on drive B. this way all you have to do is choose your start up disk in the sys preferences when you quickly need to do an edit, update a project, or whatever.
Install the OS then run all the software updates. Install FCP, open it once, quit and run all the software updates. Install the latest drivers/firmware from your capture hardware. Open FCP and begin testing.
If you decide to update an old project, make sure you duplicate it first to keep a version in 4.5, and then open the duplicated copy in FCP5. All of this is anal and perhaps a little overkill, but I have followed these practices and the upgrade has been really really smooth for me. Some of my buddies did a simple OS upgrade and they have had problems, not only with FCP, but with weird idiosyncrasies in After Effects and the like. Those problems went away when they finally bit the bullet and started with a brand new erase and install.
Also, by running this dual drive system, you can drag over any plug ins/appps/or data from your Panther drive to your Tiger drive. Once you are assured that Tiger is up and running for you, you can then erase the Panther drive and keep it for extra storage.
Good luck and hope this helps.
Jeremy
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G5 Dual 2Ghz <> 4GB RAM <> FCP 5.04 <> Kona 2
ATTO 42XS <> Huge Systems 4105 Fibre
OS 10.4.2 <> QT 7.0.3