I forget what numbers the Source engine should be putting up, but to increase your FPS, do the following:
First, make sure you have the latest drivers for your video card, and the latest version of DirectX installed. The drivers will provide the greatest increase in FPS.
Second, don't worry about FPS! As long as it looks good to you, leave it. There's no advantage to having 100+ FPS, unless you're only getting 15 or so, and it looks choppy. As long as it's smooth, it's fine. People make too big of a deal out of it, when it really doesn't make that much difference.
Finally, if you really want more FPS, then lower your display settings. Turn down all the advanced graphics features like AA and AF, it'll put less strain on your video card, thus increasing the FPS. And if it seems like it's locked at a certain FPS, like 60, then turn off Vsync in the graphics options.