It's not necessarily how much bandwidth but how often the packets are sent. A million little packets will slow down your home network, especially if it's wireless.
- If you're using wireless, try upgrading to 802.11A or 802.11N to improve the speeds of your home network.
- If you can't make that upgrade but are using wireless, try wiring the computer via ethernet (or if that's impossible, wiring the Xbox would have a similar effect). Wires are faster than wireless and will free up air traffic for the wireless devices.
...or...
- If all else fails, you could buy a cheap PC and extra network card (I bought one from Dell for $80, you could probably find one cheaper or even free) and install IPCop or m0n0wall and use it as a router. With this setup, you can manually assign how much bandwidth and priority each device on the network gets, among other nice features, and you could give priority to your dad's computer and probably still have enough bandwidth to game online without him even noticing.
If you have a common router like LinkSys, some people make custom firmware to allow you to do this, the technical name you're looking for is "Quality of Service" or "QoS." I think Belkin routers have this built in. Search Google about your router for instructions on how to edit the QoS settings for your router or see if it has them.
They also make routers that can run the software, so if you're going to upgrade your router, check this out first:
http://m0n0.ch/wall/hardware.php
