Actually what you want to do is relatively easy.
The first thing you want to do is copy the music off your iPod onto the G5. To do this I recommend you download, and install, software called iPodRip. You can get the software at
http://www.versiontracker.com. Enter the term iPodRip into the search bar at the top of the page.
Once you download and install iPodRip run it and copy all your music off the iPod and onto the Mac. When you've completed that launch iTunes and then immport all the music on the G5.
Once you've successfully done this then connect your windows-formatted iPod to your G5. I'll assume you have the iPod CD that came with the iPod? Insert that into the G5 and then install the software from the CD onto the G5.
Once you've successfully completed that then run the iPod utility and format the iPod to the Mac format.
Once you've completed that - the iPod is not reset to its factory settings and ALL files have been erased - disconnect and reconnect the iPod from the G5. iTunes should not start running and recognize the iPod. Now, import all your music from iTunes to the iPod.
