I've been playing some css on 640 x 480. all low details and I sometimes get below 90 fps in deathmatch, it can't possibly be true on a XFX 5850. I've tested now Crysis demo and I had around 40 fps but there's motion blur so it didn't played that nice. Is it just CS:S which is doing annoying or what?
fps_max 500 & switching off vsync if you hadn't done so.
At that res the gpu is hardly working. It is bound by the cpu, and the source engine is more about higher clocks than it is more cores. Try it on a higher res and see what kind of FPS you get.