Oh, you'll definitely see an improvement. I went from the x1950 pro to the HD 4850 in the spring. I had a 4600+ x2 at the time. I've just upgraded the CPU from it to a x3 phenom II 720 and saw another boost this past month. So it's not a bad path.
Hi guys just wanted to let you know i went with a secondhand HD4830 (€55, fair price) and it really helps alot. L4D went from low to high while increasing framerates and resolution. Putting the particle density to normal or low helps alot as it is the most CPU - demanding setting.
In the action now fps never goes below 50, in the old days id touch 30.