If you'd read all of the info at Sheldon's site on bb size and how they mate to specific cranks you'd do much better. What crank do you have now or what specific crank are you going to settle on? The bb as far the bike is concerned is about the shell width and threading, and it's pretty much assured you've got a 68mm shell and English threading. The bb only has to match the bike frame for specific shell width (and sometimes is simply a matter of spacers with some systems) and threading. The rest matches the crank. So, decide on a crank...maybe the old one is fine with a new ring? Doubles can often work well with a single ring on the inner position...