Your formula is correct. Pi times diameter times RPM will net you a correct SFPM. For aluminum, shoot for right around 5,000 SFPM. A 6" wheel at 3450 RPM will get you up around 5,400 SFPM which is right in the ballpark.
I cobbled up a SFPM calculator with javascript and put it here:
http://users.adelphia.net/~skiddz/Wheelspeedcalc.htm
It's ugly and kludgy, but it works. :)
There are flex shafts out there that you can run from your buffer. I've got one I run off my Jet 1hp buffer and it works great. I don't think the RotoZip would have enough torque to be very effective at buffing. I've also got one of these things and other than sink cutouts and the occasional oddball drywall cutout, it's pretty useless IMO. :)