It was around very early 2001, where I did see differences in ATI's drivers and taking things very serious. They were adding optimizations for Quake 3 engined games, ...
You know that all games have "optimizations" now - ati worked out by having tweaks for all the individual games in their drivers they could add about 10 or 20% more performance. Unfortunately having drivers that are slightly different for every game means more bugs. Nvidia followed ati's lead and now do exactly the same thing. Both sets of drivers reliability suffered because of it.
