Well, here's an option. Try Casey's Config. This will make the game look kinda bad, but will boost your FPS tremendously. Just copy the text and paste it in Steam\SteamApps\your@email.com\counter-strike source\cstrike\cfg folder. Something like that.
Here's the link if you try it out:
http://forums.pureoverclock.com/showthread.php?t=16