How can I improve Apple iTunes through my system?

I would recommend either a high-end sound card, like the ESI U24. It's external USB and works with MACs and PCs. The other option would be to send the SPDIF signal to a high end DAC. The later would probably be the best (you can spend a good $10k on a DAC if money is no object) but the ESI cards are very good.

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.

Awesome! A bunch of Mac users, G5ers to boot!

Check out stereo-link.com

First off, be weary of any ma-audio cards as they dont work in the g5's pci-x slots. Second, make sure your processor performance is set to auto or choose for me in energy saver panel as it will keep the fans dead silent. I would look at a monarchy audio or audio alchemy jitter filter, they will help as the digital out is so much better. Try connecting the g5 to a power conditioner or the same ground as your stereo. If the serial ATA drives get to loud then try a external firewire drive that you can place out of the way. I have also had good luck with placing a older gigabit ethernet g4 in another room and running full gigabit to the g5, do not really have any bandwidth issues for movies or music and don't hear the 10k scsi drives spinning away with the g4.