The SU800 can control three remote groups. The D200, D300, and D700 (per specs, I haven't handled a D700) can control only two groups.
Yes, the SU800 can only be a commander as it cannot function as a flash as well, as can the SB800 and SB900. If you are not planning to use on-camera flash, the SU800 is smaller and lighter, and I prefer to us it. If I need a light from near camera position, I usually bring in another SB800.