The batteries I am using are Optima Yellow Tops. They are 12 volt, and come in a couple sizes, I am using three of the 55 AH version.
My controller is a 24-48 volt Alltrax, so I could use one more battery if I could figure out how to get it in there.
I figure my cycle would go 45 mph on a 48 volt system. Keep in mind gearing and weight are factors.
Lots of guys have been doing 72V crotch rockets. Sport bikes have light frames and are pretty good about mounting batteries.
If you haven't already, get a copy of "Secrets of EL Ninja". It is a pretty good resource. Lots of information together under one cover.
Also check out the EV Album:
http://www.evalbum.com/type/MTCY for electric motorcycles. It lists the specs for each bike. Find a couple that have a 48V system and look to see what the speed listed is.