Well, you have a machine that can be better with a few tweaks.
Install a second harddrive
Install more RAM
Install a processor upgrade
Install a faster graphics card
Usually when you get the stuff to install, it has some info on how to do it, but it's pretty straight forward and if you've already installed RAM and the Zip-drive it's pretty much the same when installing a second harddrive, just connect the things correctly.
When it comes to processor upgrades and graphics cards, it's a bit more tricky, but the stationary G4's that you have is so easy to work with. Only important thing is more or less to remember to ground yourself so you don't damage any of the sensitive circuts.
If you don't have room for a second harddrive, you can always transfer your old to a new one and install a 200 GB harddrive or something like that. That should keep you happy for a while.
Installing more RAM, even if you have a bit, is always great, especially if you want to run OS X on it. Aim for 512 MB or more.
The processor upgrade is the big thing... that will cost you a bit, depending on how far you want to go (I think a 1.5 GHz is around $500 USD). Some processor upgrades also allows you to upgrade your machine to a G5-processor, but having a G4 at 1 GHz would most likely make you very happy.
Not sure if that as much help, but calculate on how much it would cost to make your PowerMac more powerful compared to a new iMac. If the difference isn't too big, then buy a new one and don't care of the money you get for the PowerMac, that you can use to pay for more RAM in the future.