Are you looking to buy new or used? I had a Nikon D40 but ended up selling it and getting a used Nikon D70. I like the camera, but I have to admit that I don't know much about the differences between the different models and stuff - DH knows way more about that kind of stuff, and is more into it then I am, and I just do what he suggests. I know he'll have some really good information for you, so I'll email him this and then will post his response.
Okay, well DH is pretty long winded so I thought he'd have lots to say but he just said: Nikon D40. Easily the best deal to be had under $700. Check out http://www.kenrockwell.com/nikon/d40.htm.
Check out that Ken Rockwell site for info on other cameras and lenses as well - he has tons of reviews.