I haven't found any drivers that do very much for either nVidia or AMD cards. Most are all performance increase related, and the only one I can think of that would increase the graphics would be Omegas drivers, however I haven't tried his latest drivers to confirm this. Mind you, in-game there are tweaks you can do from the console to increase the graphics a bit (I have them bound to a macro to make it really easy to kick them in). All they really do is increase foliage, increase distance.. things like that. Take a look at
THIS thread on the WoW forums. First post has the tweaks, and below it is the ones to reset them. If you get any lag in the game, decrease the foliage one (groundEffectDensity) to something like 175 (I find this # to be a good balance).
My advice is to bind them as a macro, because most of them reset when you close the game.
Oh, and it shows CharacterAmbient as having no value, but the tweak is to set it to 0 (default 1).