I'd agree with the above advice as both OS X and CS3 are easy installs. The only comment I wanted to make is I noticed you said you have a G4 which would make your machine a few years old now. If you are running CS2 acceptably, then you should be able to run CS3 relatively well also.
I would just want to double check OS X.5 first though, as more than past versions of OS X, it really likes to have a significant amount of RAM to run properly and remember Apple is now concentrating these OS X updates on the Intel platform with dual processors. Just make sure your G4 is included in the acceptable machines for it to run, and I'd suggest 2GB RAM to run it properly or if you have an older iBook I seem to remember they would allow upto 1.25GB of RAM so get your full amount. Anything less than 1GB, I wouldn't personally bother with Leopard as I think you may find it a little frustrating so I'd just stick with what you have now and wait until you next get a new laptop/desktop in the future for Leopard.