800x600 places all the load on the CPU. The GPU will have nothing to do. Play at a higher resolution and see what happens.
Having said that, your PC shouldn't have trouble running CS: Source at ANY resolution. What power supply do you have?
dont remember what company but its a 750w
Changed from 800x600 to 1152x864 that is my desktop res.
i had higher fps drop with the 1152x684