SB400, SB600, SB800. All will work nicely. If your photographic interest compel you beyond amateur status, being that the D40 does not afford you "command" control for multiple flash units, I'd recommend the SB 800. It's enables one to operate and control more than one flash at any given time. This suggestion is of course based upon the assumption that you will not for some time be willing to up-grade cameras.