I hear rumblings of a lot of members bitching about sites with Flash because they can't (easily) download the content. If that doesn't bother you (and I'm not at all convinced it's a really significant issue) then I'd do 1 or 1.5Mbit Flash, and maybe 256 and 512K. Honestly, I think the surfers who are still slower than 256k are few and far between enough that they're used to waiting forever for their content to download.

On the other hand, if your encoding is automated, you could probably pick up a few extra customers having a 56K stream, but the quality is going to be so bad I'm not even sure it's worth it.