If you are running OS 10.5 you may want to upgrade to VW 2008, in the other hand if your PowerMac G4 is printing in Mac OS X, share the printer and try it from one of your new iMac's. Also, try to emulate the drivers using HP 800 or even HP 450C from the printer list(OS X). I could not find any printer driver support for Mac OS X 10.4 or 10.5 for HP DJ 500.
Juan P.