i did some tests with an old nvidia card 2 years back and tried lads of different drivers to see what effect it would have on my 3d benchmarks
and as a general rule of thumb, the older the driver, the higher my score, until i hit a point that gave no extra gains, i saved this as a zip file for future use, not sure of the ATI stuff is the same?
it was explained to me, that the older drivers are more "streamlined" and not bogged down with newer features that would never benefit my lower spec card