Ok, update for you.
My system, is to say the least, Odd. I believe I am getting bottlenecked by the fact that its only allowing AGP 4x and not 8x.
Sooo, whenever I turned it on low, it really didn't have much of an effect ( a whole 5 fps on average). Then, I turn it onto DX 8.1, and nothing happens in terms of FPS.
So once again, its up to you, but I don't see you having to have it set to lower DX levels.