Going from a GF2 to a 9600, you got a much faster, more powerful videocard.
Unfortunately, you also went from a DirectX 7.0 to a DirectX 9.0 videocard. DX9.0 has tons more features and whizbangs which can easily consume-up all that added new graphics horsepower.. as well as place substantially more load on the rest of your PC to compensate.
Luckily, Halflife2/CounterStrike:Source allows you to force the DirectX level with the mat_dxlevel command. You can put this in the console as:
mat_dxlevel 70 (for DirectX 7.0 as your GF2 used to supply)
mat_dxlevel 80 (DirectX 8.0)
mat_dxlevel 81 (DirectX 8.1)
Realize the newer, cooler walls and water will go back to looking how they did on the GF2. The extra nice looking surfaces, shaders and the like take a large performance toll, but can be disabled by forcing the older DX level compliance.
Edit- a small visual example using HL2:Source LostCoast:
(click to see the full-size image)
DirectX 7.0 level (Geforce2):
DirectX 9.0 level (9600 series):
