You can use up to version 6.x of FCP ( included in Final Cut Studio 2) in your machine.
You can use up to OSX Leopard (not Snow Leopard) on your machine.
My previous computer was a dual 2.0 G5, and I was running Leopard and FInal Cut Studio 2 on it. My computer was older and slower than yours. I would configure the same on your machine. Trying to figure out the older version combinations is an incredible pain. FCP version 6.x is solid and pretty close in features to the current version.
You must realize that the last G5 tower was sold 6 years ago and most people who edit professioally have moved on to Intel. Our memories might be a little fuzzy about specific OS/FCP versions. There have been many updates over the years and they kind of blend together.