It depends on what kind of game you want to program. I like multiplayer on line games like:

-Wolfenstein: Enemy Territory (already there)
-America's Army (800-900 megs http://www.americasarmy.com/ )
-PlaneShift (MMPO http://www.planeshift.it/download.html )
-Cube (http://www.cubeengine.com/ ) -these guys encourage source developement
-X-pilot (fairly low-res, simple)
-BZ flag (also fairly simple)
-Paradise (netrek - http://www.netrek.org/ )

You should probably start with something simple, download the source code, and make little changes to it and see if they work to get an idea of whats involved before you start on a full-fledged game from scratch.

Moustache