One possible reason you're getting a low framerate could be the fact that older Source games (pre Orange Box) only support use of single cores. So it is only using one of the 2.6 GHz cores. Also Source is more reliant on CPU than GPU.
However I still think you should be getting a higher framerate, even with one core being used. If you have vertical sync enabled, it will lock your framerate to the same speed as the refresh rate of your monitor. Some people recommend leaving it enabled, but if you want a higher FPS disable it.