Ati Radeon HD 4670 which best drivers ?

I had to reinstall Win 7 64 Bit on my Imac 27 (Model Late dec 09). This is my second install and while everything goes well if you follow the obvious tips I have now a issue with the pictures quality of the display. I can see a drop in sharpness and overall quality picture on everything from watching movies, photos to even surfing the internet etc.. I am able to compare from things I have previously watched. I would estimate the drop in quality is about 20 %.
My Card is ATI Mobility Radeon HD 4670
After installing win 7 64bit and using drivers from snow leopard Boot camp I have
ATI Driver Version 8.661
After upgrading to boot camp 3.1 : ATI Driver 8.681
After installing drivers from Radeon website I have
ATI Driver Version 8.821
None of these drivers are fixing the quality problem.
I did try also Boot camp version 3.2, same issue.
I can't remember what drivers i've used last year when I did my 1st install
Does anyone experienced something similar and be able to fix it.
This is very tricky because I guess if it would have been my 1st install I would have probably never realised this drop of qualitypn the display. I would have assumed the picture quality I see is the picture i'm suppose to get.