So, you want to have the PowerMac G4 connected to your campus network/internet and to:
i. Hold a larger iTunes music library
ii. Share internet to your PowerBook
iii. Store movies and other media types so that your PowerBook doesn't have to?
If this is correct then you'll need:
1. Some way of connecting the two machines, probably wireless. 802.11b will be too slow for your needs, but unfortunately that PM you've bought fits an old Airport (802.11b) card. Belkin do a USB2 802.11g dongle but your Mac has USB1 (therefore, all USB WiFi dongles are not going to work at full speed). Look for PCI like
this one (expensive though). Your PowerBook will also need an Airport Extreme card (assuming it's a relatively new PowerBook).
2. iTunes libraries are easily shared. You can keep a cut down version on your PowerBook so while you are on the move you can still listen to your music but then also have a much larger library on the PowerMac. To share just turn sharing on in the PM's Preferences and it'll appear as a playlist on your PowerBook (if not, make sure you have 'look for shared music' clicked in preferences).
3.Sharing internet is easy. Once the wireless device is installed, go to the PowerMac's System Preferences -> Sharing -> Internet and start sharing your ethernet connection with your wireless connection. Then get the PowerBook to connect to the newly created ad hoc wireless connection the PowerMac has just created.
4. So far, everything above would just require a 802.11b wireless connection, which is easy to install as all you'd need is two airport cards. However, I wouldn't recommend opening movies across wireless at all as in my experience things stutter. If you must, make sure the files you're opening are not the highest quality in the world. E.g. No HD content. To do this, for accessibility, put all your movies into one folder (may i recommend the 'Movies' folder in your home folder
) then make sure File sharing is on in the System Preferences -> Sharing tab. On your PowerBook click 'Network' on a Finder sidebar or 'Go' Menu -> Network at the top of your screen. The PowerMac should appear, click this, log on and it'll ask which drive you want to mount. If the movies are in your movies folder then mount your home folder (most likely your name) and you'll be able to access your movies from there.
Personally I would use Ethernet but that defeats the point of having a laptop.