I've never had the opportunity to actually use lights at a ceremony or reception (unless I'm doing the formals at the reception venue, in which case the guests aren't normally there), but if I did it would be completely dependant on the venue. In my experience, there are just too many people and too much movement to make the use of strobes feasible. Add to this the fact that you would likely be ruining, or at least degrading the atmosphere for the couple and guests...
Is this just a general question, or do you have a situation to work with?
A couple of flash heads aimed at the ceiling, walls, or ceiling corner areas, can be used to boost ambient light levels. You need radio triggers to prevent P&Sers from tripping your lights. Two, 400 watt-second flash heads can boost the light up amazingly, if they are positioned right. This is not always practical....in the past I relied upon the Sunpak 622 Super, which was for a long time, the MOST-powerful handle-mount flash ever made, and about the same actual power (in real-world f/stop required) as many 400 watt-second monolights, due to the efficiency of its very large reflector and Fresnel lens.
Before monolights got cheap, Sunpak 622's used to be a location lighting staple...LOADS of power, from 4 C-cell batteries...Zoomed to TELE, the ceiling bounce possible with a 622 rivals that of the Norman 400B or the Lumedyne flashes costing $1100.
Anyway..."it depends". In a smaller room,with lower ceilings, two SB 800's will provide some more light, but nowhere near say two 400 w-s lights bounced. A higher-priced speedlight like a SB 800 or 580 EX-II (both are ~ equal in power) is nowhere near as powerful as a 400 watt-second monolight. THe watt-second equivalent is more like 80 watt-seconds, given equal beam spread, and a monolight or pack and head flash can easily be fitted with a high-efficiency reflector in the 11 to 11.5 inch category, which will throw a wide spread of light AND have vastly more power than a speedlight. Speedlights have higher guide numbers as they are zoomed out to very narrow-angle reflector settings that only cover a very small angle.
Umbrellas will not produce soft light over large areas from long distances...so...they either will be useful and helpful, or not: once again "it depends" on how close they are to the shooting area.
I think if I had only two SB 800's, I'd concentrate on using them to light only the localized shooting area, instead of trying to overextend them. THis is an area where a CHEAP, Speedotron Brown Line D402 400 watt-second flash generator and two heads for $295 from eBay would make a heck of a lot more sense than six $400 SB-800 flash units...and would produce more and better quality light output...
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.