hey, thinkricky, check out strobist.com's Lighting 101.
After comments and advice from several TPF'ers, I am down to deciding between the flash mentioned on strobist's Lighting 101--the LumoPro LP160--which, at $160, I could afford to get in the next month or so, and the SB700, which I'd probably try to find used, as it will be several months before I could spring for a new one.
I admit though, I'm still a bit confused by it all, and haven't had time to do the research to understand things better. So, I might be better off waiting until after February when I'm only doing ONE full-time job instead of a full-time real job and a 30-hr a week contract job.
What I haven't yet wrapped my head around is TTL vs Manual, how that plays into slave and commander modes, and how ALL of that would work, or NOT work, with my D5100.
IF I have understood it correctly, though, I am leaning toward the SB700, because I could use it off camera manually, or ON camera, by itself, OR as a commander flash to control a second, slave flash off-camera (but does the slave flash ALSO have to be the same brand??). On the other hand, maybe I should go ahead and just get the LumoPro, since I can't afford more than that anyway right now, and THEN get the SB700 later...
Good luck! If you decide what you're getting, let me know!! What kind of camera do you have, by the way?