nVidia stopped adding anything for the 6xxx cards after 84.25 (early 2006), and some of the things in newer late 2006, early 07 Forceware packages conflicted with the 6200 and the 6600 cards, including yours. I have heard that there was an "81.xx" driver that was supposed to be better, and the 84.25 was never certified as WHQL (it remains a Beta), although it worked just fine for my own 6600 GT.
Looking at my own copies of back-dated Forceware, the 81.98, which was also the final W9x driver, may have been the same Forceware described as being optimal by a couple of hardware advisory articles I've seen, like this one at nZone:
http://forums.nvidia.com/index.php?showtopic=1029