Even if new of this forum (
hello, how are you 
) I am a long time user of FS9, and in the olden times when FS9 went out in the shops I had a Athlon Xp 1700 with the same video-card you have, with which I never had any problems under FS2002.
Sadly enough, I discovered there was no way in hell to have a satisfying haze at the horizon with the Rad 9250 back in the days, and I had to resolve it... buying a Gefo TI4400. :-/
It may not look too helpful, my writ, but it's life experience. To be constructive, if, like you write, putting the visibility to 3 miles works to give you a haze, and you can't get another video-card, that's what I would do in your shoes. After all, in populated areas, rarely there's more visibility nowadays. So, unless you're a bush plane pilot, you'd be set.

That is, unless someone that knows more than me (
I'm hardly infallible) manages to find you a better solution.

There is no such a thing as overkill. Only unworthy targets.