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What kind of G4 do you own, and what is the CPU speed? Leopard requires a minimum 867 mhz processor, no exceptions (as per the installer). So if you aren't currently running 867 mhz or faster, then it looks like you'll be going with Tiger. In my honest opinion, even if your computer is faster than 867 mhz, I would still recommend going with Tiger unless there is something you really need that Leopard offers. Tiger is well suited for the G4, and will run great and supports mostly everything. I say stick with Tiger unless you have a G5 or Intel Mac. Of course, that's just my opinion.
It is always recommended that you upgrade your RAM, although it isn't necessarily required. But one of the best upgrades you can make is to max out your RAM. This will be more noticable than any other upgrade you can make (other than a large processor upgrade). So I personally recommend it if you are willing to go for it.