An 64-bit OS is not required, but Flash CS5 runs *very* fine on Win7 64 at least. Just ask me ;)
In the CS5 release, only After Effects actually requires a 64-bit OS. Buyers beware!
Flash does not have extreme memory demands, so 64-bit versus 32-bit does not make that much of a difference.
Not everything that can be counted counts, and not everything that counts can be counted.