Retrospect will handle this for you very nicely indeed. Like you, I have my entire music library on both my G5 and my MacBook Pro. I add new music (import and download) only to my G5, and have Retrospect run a weekly script to update my MacBook Pro music library with any new files added on the G5. It runs on the network, so as long as my MacBook Pro is on and awake, it just does it without me having to do anything. Best of all, if you tell Retrospect that the source is the entire iTunes music folder and the target the same folder on your laptop, it copies across all your playlists, playcounts, and other prefs too.
One thing you should be aware of: If you have a big music library (mine is more than 40 GB) you should run the first sync on a hardwired network connection, otherwise it'll take all day over the wireless link. Thereafter, updating doesn't take too long. On average, I add about 30-40 tracks a week to my G5; it's all done and dusted on a Saturday morning over the wireless link in about 4 minutes flat.