In my Quake II & III days, I had an older ATI that did outstanding fog & haze, much neater than what I have experienced on my Nvidia's (2 diff cards) .
Granted the sims are different games than 1st person shooters: does ATI do better haze/fog than nVidia? Or it a matter of OpenGL doing fog/haze better than DirectX?
Is the trouble with ATI 9800 & 2004 generally solved (through the latest drivers? 3.9?) Before the one on Christmas Eve (a 5950), I purchased an nVidia card over an ATI (maybe the 8500 two years back) after extraordinary 2002 hassles. I don't want to repeat the affair now. But hey... if there is a gain to made for the same price I'd already paid... :)