What model Nikon SLR to buy, relatively inexpensive?

I've decided that I need a digital SLR for school. After reading reviews on the internet and using some of my friends' cameras, I've decided to go with Nikon. But I can't decide which model. Obviously I'm on a budget since I'm a student, and I'll have to buy a used one. Can anyone give me reasons in plain black and white, which would be better- d50, d70/d70s, or d40. From what I can tell, the d90 is out of my price range, even used. I'm wanting to stay below $400. Also, the type of photography I need it for is high resolution, high contrast documentation (I'm an architecture student). But I also want something that is versatile and can be used for outdoors and sports.

You can get a D60, D70, D80, and D3000 for that price. They will be well used but still take good pictures. You may also want to consider a P7000. While a small sensor, it has very good image quality and can be found new. In general all of these cameras should meet your basic needs.

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Take a good look at the Nikion D5100.It has a lot more features than a lot of those cameras listed, plus it has a really good price. I have the d5000 and am completely happy with it. It takes amazing pictures and is full of great features. Its really a great starter camera that is still advanced enough for even the pros to enjoy. Also, if you check out kenrockwell.com he reviews each camera and gives you a really easy way of figuring out what you want. I would read up on all those you are looking at and then take a look at the d5000 or the d5100. Hope this helps. Now, for 400 dollars you probably can get a d5000 used off cragislist. I did!

Out of the ones you mentioned I'd opt for the d70s and second the d50.