I bought Easy FLV
http://www.easyflv.com/ because I had no real idea what I was doing with flash movies and it made it possible to just start using them without trying too hard.
That said, there are probably other free or cheaper bits of software out there that will convert flash videos for you and the results may be better.
In terms of bandwidth, I think flash vids have smaller file sizes.
And people are used to flash vids thanks to Youtube.