I'm looking into the same thing too Mike.
I've got my eye on the 580EX.
The flash units fit on the hot shoe.
They need their own batteries.
I would recommend a diffuser, it softens and spreads the light giving a more natural feel. Don't need one straight away but it is good to have one eventually. Also you can make your own until you get the realy one.
The diffuser I like is the "
Sto-Fen Omnibounce" available here in Australia from Film & Video Extras Pty Ltd 02-9618-3104.
sales@fvextras.com
Not sure of the cost.
I'm told the 580EX is a good unit and has an LCD screen on the back giving you important info on the settings it's using. It might be overkill for our applications but it's in the higher end of flash units.
I asked about the 580EX yesterday at G&V imaging and Ned told me to go and see him personally.
I don't know about any other models Mike, maybe someone else can give a better suggestion.