I'm running four 48" T5 bulbs on an Icecap 660. I think you can run 2,3 or 4 bulbs, but 4 is the most common. From memory, I think that a 660 pushed four 48" tubes at 80W.
They're DARN bright with the SLS reflectors. I've got them over a 120G tank and the 4x54 (80W?) bulbs are FAR brighter...