SBA was/is a feature that was never really utilized to any appreciable degree. Many of the higher performance cards began dropping it altogether. Except for the Creative model, the original GeForce 256 was one of the first that didn't support it.
Essentially, the memory addresses are increased from 32 to 40. More addresses meant that more processes can be carried out simultaneously. In theory, this meant an increase in performance. In the real world, there were practically none outside of benchmarks, and even then, the gain was very minimal. What it did do, was increase instability. Especially with the advent of the original AMD K7 series and Irongate motherboards (and to a lesser degree, VIA's KX133).
For it to work, it needs to be supported in the video card, the drivers, and motherboard. If any one in the chain has it disabled, SBA will not work. I wouldn't worry about it. If it's showing up as disabled for you, just leave it. You won't miss it at all.