Like a whole host of people that I have talked to over the last week told me, go to www.strobist.com
and read how to do what you are thinking of. Start at Lighting 101 and go from there!
I personally think that you could get one strobe(monolight) with either an umbrella or softbox and be good to go with a flash on the other side and be just fine. you might have to work with how the strobe fires etc to get the fill that you want, however I think you could do that without too much of a problem. Most strobes will have the ability to fire when the "see" a flash as well, so that will help you too...
My whole thing was that I'm starting to do a lot of portrait work and I am needing more coverage and better light for those posed shots, so I bought the two 800s. I'm not sure what you are looking at doing, but that's the decision that I came to after literally two weeks of research and looking at different systems on the Net. Hope this helps!