<<<g5 dual ghz has >>>
How many GHz?
<<< 100% memory to fcp.>>>
How did you determine that? Did you force it to be that way? That might be part of the problem. If OSX needs to do drive housekeeping outside of FCP and it can't, you will get errors like that. You may be running Final Cut, but the machine has to manage a lot of other things at the same time.
<<<g-raid 800gb via 800 firewire>>>
Anything else connected to the 400 port at the same time? All the FireWire connections to a Mac run on the same chip set and have to share resources. the only way to prevent this is to get an 800 FireWire card and plug the G-Raid in separate from the Mac's built in connections. Disconnect everything else from the Mac and try again.
Do you ever get the Spinning Beach Ball Of Death?
You shouldn't have problems with 8G of RAM, but are you trying to open a 2-hour show with two video and a billion audio tracks?
That happened to someone (with less memory) further down the forum. They seemed to have a very large machine until you did the arithmetic of how many resources were being called.
Koz