Lan Game Reviews has a good how-to guide for seting up a router that will allow you to play online games no matter who is on your network. They use m0n0wall for the LiveCD, no hard drive needed.
The next step is to enable Traffic Shaping. This is what prioritizes packets so you get great pings while downloading. Click on the Traffic Shaping button on the left menu bar, and click the Magic Shaper Wizard tab. Select the checkbox saying Set P2P traffic to lowest priority and input the downstream and upstream speeds for your connection.