Yes your startup disk or your boot disk... however you want to put it, it is the disk that your G5 came with, that holds the OS and your apps. Install FCP on that disk. It would be a bad idea to install it on your Medea G-Raid drive since you want the outer most parts of the array to be free for your DV or whatever codec you'll be using. Tweaks for FCP involving other disks than the boot drive could include moving your scratch volumes to a second drive on it's own channel... once again preferably a drive that's neither your primary editing drive nor your boot drive. Perhaps a second internal sata drive other than the boot drive.