Do you by any chance belong to the Yahoo! group fanfic_law? Someone there could probably help you. If you don't, I can ask for you; I think I have a few things of my own to ask them anyway.

For now, as much as I HATE wikipedia, its article on NYPD ranks seems to jive pretty well with what I vaguely recall from...hmm, maybe editing an SVU fic? Anyway, I figure that might give you a place to start, at least, so here's that link: http://en.wikipedia.org/wiki/NYPD#Ranks_of_the_NYPD

Good luck! :-)