FCP, in general, is pretty much real time for most functionality — as long as you have the drive speed and RAM to support it.
On a G5, using a second internal SATA drive, it’s practical to expect RT performance for most single channel (even dual) 8-bit uncompressed work. As you add layers, you’ll need more bandwidth, possibly a RAID of some sort. 4GB RAM would not be too much.
Color Finesse has a Mac version that does real-time correction with files bounced via XML, though it might be version 2, not 1.5.
On the whole, for most normal workflow, FCP has little waiting for renders while editing, though you’re probably safe to assume a full render for output, but even that’s fading fast in FCP5.
JB
Blue Smoke & Mirrors Ltd.
FCP Finishing
Xsan Integration
310-292-0416