I used to use a NAS system (well Linux with SMB/NFS but that is what most NAS boxes use anyway) for my iTunes folder to my iMac. In the end I moved the iTunes to my Mac as I had to continually connect to the network share before firing up iTunes, else it would default to my Music folder.
You could, I guess, use ‘Back to my mac’ if you are running leopard on both machines. Or share out your mac’s Music folder using SMB.
Make sure that any NAS box you buy will work with MacOS. I have heard that some only work with Windows, must be a strange network protocol…