I've seen the stutter when rolling in every version of AH since it was in beta. I showed it to HT at a con waaaay back when a voodoo5 was a fast card, but I don't know if he ever was able to make an effort to work on it. I suspected it was a side effect of the game's internal refresh rate (in the low 30s if I recall correctly), the actual framerate, the rendering buffer, and the refresh rate of the monitor, but of course I'll never know since I'm not an HTC programmer
To more directly answer your question, I still saw the stuttering when rolling (almost like having double vision) when I got my new Nvidia 6800GT even when my framerates were above 40. I only got a 10-20% framerate increase going from my GF4-4200 to the 6800GT but I was able to add 4xFSAA and 4x ansio at 1280x1024 and lose almost no framerate unless I started turning the detail sliders way up.
So going from a 9800 to a 6800GT or X800, you will probably get an image quality increase and maybe a bit of a framerate increase, but you'll probably need to also get a cpu upgrade to something really fast if you want to see big framerate increases.
In the meantime, I suggest running the sliders pretty far down and adjusting screen resolution until you're happy with the framerates in furballs and near burning fields. A new video card will look nice and IMHO a 6800GT would be a reasonable upgrade for you and not a waste of money, but it would be held back somewhat by the rest of your system until you matched it with an athlon 64 or top end P4. You can also turn the texture size down and fiddle with the texture preload, but everyone seems to get different results with the preload so you'll just have to try it out to see if it helps or not.
