I'm in a very similar situation, I have a D40 and mainly use Nikkors 18-200mm and 35mm DX. And I don't have much to spend on cameras right now.
But a used D90 seems like a good deal right now to me, as many D90 users are upgrading to D7000. I think a good used D90 could be had for around $600.
A D90 will give me 1) autobracketing (currently I manually bracket fairly often) 2) autofocus motor (I have a 50mm 1.8 AF-D I would use as a portrait style lens) 3) the extra pixels - I like to print 8x10, and sometimes after a little cropping 6 megs is a little rough.
(OTOH, I could use a new printer, and a wide angle lens would be useful)