I usually set up one B800 or B400 (Alienbee) light for a reception hall, but sometimes I'll use two. The location and direction will depend on the layout & design of the hall. My favorite is usually when the hall has a balcony where I can set it up and point it down.
But because that creates a directional light source, I also use on-camera flash much of the time as well. This way, I can use the remote light as a main light and the on-camera as fill....or vice versa if I adjust the exposure as such.
I also really like positioning myself so that the remote light is behind the subject, so that they are back lit, then use the on-camera to light up the front.
As Derrell said, using an umbrella will depend on the situation. If your light is 20 feet away, the umbrella really won't make much of a difference on how soft the light is, but it will still eat up a lot of power. It can, however, spread the light around the room...but it's likely that you can also do that by bouncing the light off the ceiling or wall etc.
So really, what you'd need to do, is get into the room and assess what you have to work with and make due with what you have.
And yes, I do think that a the SB-800 has enough power for something like this...but keep in mind that when using a battery powered flashes, you have to be aware of what your battery levels are at and you have to be aware of your recycle times so that you don't get caught with no flash at a key moment.
That's what I love about using a studio light...I plug it in and it's good to go all night.