Is it possible to upgrade a 2001 G4 733 from System 9.2 to newest Leopard version? If it's possible, is it worth it or am I better off buying another computer with the latest Leopard?
This lists the absolute minimum specs for Mac OS X 10.5 http://www.apple.com/macosx/techspecs/
You may be able to tweak it to install it despite the system being much slower... if you want to upgrade that system to Mac OS X, 10.3 would probably be the best for your hardware, still needing as much RAM as the hardware can handle and you can afford. I wouldn't recommend 10.5 for your system even if you managed to install it there - if you need 10.5 for some programs, you'll be better off getting a new(er) machine for that. Your Mac is otherwise still great though, it would be a pity to get rid of it.
You Mac isn't officially supported by Leopard, but that doesn't mean that you can't get it on there. Still, you would lose the ability to run Classic mode since Leopard dropped support for Classic. That Mac should be able to dual-boot between OS 9 and Leopard, but you won't have any Classic mode available as you did with previous versions of OS X.