Okay so I've had my doubts about the ATI Drivers lately, they seem to either bring new bugs or lessen performance, so I've done some tests today, it took about an hour or two to do all of these but I think the end result was worth it. Keep in mind these might not be the same results for you as I'm using an older 3 Series (3850, 3870, 3870 X2, etc) Video Card. In the next few months when OpenCL is supported more you'll want the Latest ATI drivers or at least 9.11 for OpenCL support for your GPU.
I tested 5 drivers but only the results between 9.9 and 9.11 will be displayed, all the other drivers suffered from reduced performance ( 7.12 / Omega Drivers, 9.3, 9.7 and 9.10)
Clean (ATI Software Free) Windows XP SP3
Restart in Safe Mode & Uninstall Old Drivers
Restart in Normal Windows & Install New Drivers
That's what I did for 4 different ATI Drivers (I only needed to know some of the recent benchmarks on the latest ATI Drivers) anyway here are my Results below. Also before you read the below I was only testing this for Source Engine Games, so don't get your hopes up about other games ever being tested by me, it takes to much damn time.
Team Fortress 2
Left 4 Dead
ATI Drivers Version 9.9
ATI Drivers Version 9.11
Anyway you look at it, the 9.9 Drivers seem to have somewhat better FPS average but a difference of max being 4 - 5 FPS there isn't much difference, the testing rarely dropped below 60 FPS so none of this will make any difference for me. Thought I'd just share for you performance junkies that might want to test the different drivers yourselves. The 9.11 drivers seem to offer a more reliant FPS average as the large drops I saw with the 9.9 drivers are now either smaller dips or performance improvements, they both have there flaws and there both about even.
Higher Max FPS: 9.9 (by about 5 FPS)
Higher Min FPS: 9.9 (by about 3 FPS)
Best Average FPS: It's a tie, there's very little performance difference between these drivers, I'm going to use 9.11 though as it has added support for OpenCL.
PS: ATI Drivers v9.11 are currently in Beta Testing.