One word - processor.
I had a slightly similar problem to yours.
I had a 6200 Nvidia PCI, 512mb ram, and a P4 2.4. I tried upgrading ram, didn't work. Bought a whole new computer and it was a P4 3.0ghz and even with 2 gigs of ram and a ATI 1950x GT andmy fps would go down to the 20's. I actually bought an e6750 core 2 duo today and I checked my fps on source and it's a consistent 60.
Wow. I had a P43.2GHz, 2GB DDR800 and a X850XT PE and I never had that kind of problem when playing HL2 Death Match or CS:S. And when I upgraded to a P4 3.4GHzEE it got better. And my new system TF2 will dip to 70 and jump over 250+ and average around 150FPS and thats at 1280x1024.
His main problem if the CPU more than the VGA as Source is very CPU intensive. But yea an overall new PC will do you wounders in CS:S