20 - 30 fps in CS:S huh?
I suppose you are running it on high, aren't you?
Disable HDR and put any setting (doesn't matter models, textures or effects) to medium. Also GameXP helped my friend, he had a 6200A, played 40-70fps on Medium/800x600/2xAA.
For your 6600GT and proc I recommend
Models: High/Medium(for good fps)
Effects: Medium/Low (effects don't matter that much)
Water: Reflect World
HDR: None/low-level (HDR slows it all down, really)
Res: 800x600 (1024x768 if you put the lowest settings I recommend)
Your processor is limiting the card alot!
My crappy Celeron 1.7 @ 2.0 is limiting even a FX5500...
GFX doesn't matter as much as your FPS, you bought the game
to play it, not to look at it right?