DOH! 😢
It occurs to me --finally-- why you may be seeing this slowness on a wried connection. That latest G4 iMacs retained the 10/100BASE ethernet port Apple used to put on all consumer and educational Macs. The PowerMacs and PowerBook were considered "pro" models and had 10/100/1000BASE ethernet ports--much faster.
You may see improvement by going wireless if your iMac has USB 2.0 ports:
NewerTech MAXPower 802.11n/g/b High-Performance Wireles USB Adapter w/bonus ext cradle. For Mac & PC
It will give wireless "n" speeds if used in a USB 2.0 port--that's the protocol your other devices are probably using.
To see if you have USB 2.0 ports, hold OPTION and, while hlding it, click the Apple menu. In 10.5, the first entry should change to "System Profiler." Launch it.
In the opening screen:
find the code in the second line that I've indicated with the red arrow; for a G4 iMac the line will be "PowerMacx,x"--please post your code and your screen size (15, 17, or 20). From that we can find out if you have USB 2, making the wireless option viable.