Good place to start is wikipedia - search for potato cannon. In there, there is a link to here:
http://www1.iwvisp.com/thehalls/ggdt/ where you can get software to calculate theoretical velocities and stuff. He also has a page showing how to modify a sprinkler valve for rapid discharge (for manual firing), and pages discussing different designs of spud guns and paintball guns.
Make sure you read various safety warnings about PVC guns! Use only pressure-rated pipe, prep the pipe and use primer, allow the cement to cure for at least 24 hours and don't use them in hot or cold weather.
You could probably modify a sprinkler for pneumatic opening, then use a 12 volt air valve from one of the old pneumatics kits as the trigger instead of the solenoid.
Our latest toy is a 20 lb. propane tank connected to a 1 inch pipe. Overkill, yes, but shooting a carrot at something solid makes instant carrot juice. More like orange fog.