How can I improve Apple iTunes through my system?

Brimac, there is still an issue with the pci slots in the entry level G5 since it only accepts 3.3 volt cards. Also the G5 is very quiet, espescially compared to late model G4. It can sit in the listening room and not draw any attention to itself.

Go to preferences and turn off all the effects. Leave the volume control on the main screen all the way up (to the right) and don't change the volume or add EQ for individual songs by using the Info/Option control. Since the G5 comes with a built in optical digital out, you should connect it to an external high quality D/A. If you want, you can add a jitter reduction device between the G5 and the D/A.