The great thing about Nikon stuff is it's made to last, so buying used is not that big of a deal. Here's some various models you might be interested in.
The Nikon D3XXX and D5XXX series are geared towards hobbyists and infrequent users. These cameras will take great pictures, but the autofocus speed is lacking and options are primarily selected via a menu system rather than by buttons. You can pick up a used D3100, with an 18-55 lens for around $300.
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The D5000 is a bit of a step up from the D3100 and costs a bit more. I have one that I've converted to infrared.
http://www.adorama.com/US%20%20%20%20591655.html
The Nikon D90 has been out for a bit longer, but is a very nice camera. It's geared towards the more serious amateur market. The autofocus speed is better and most of the options are available pretty quickly via various option buttons on the camera. You can get one without the lens for a bit north of $350.
http://www.adorama.com/US%20%20%20%20587417.html
The D300 is similar to the D90, but is geared a bit more towards the serious amateur and professional market.
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These models have various degrees of video capabilities, so if that is important you will want to compare the various models.
As far as lenses go, you'll probably want to start out with an 18-55 lens which is actually quite nice optically and provides almost macro level close up ability. You can pick one up used for about $75.
http://www.adorama.com/US%20%20%20%20594222.html
A step up would be something like a 24-120 or a 18-70.
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An 18-70 paired with a 70-300 would be a pretty nice lens combination.
http://www.adorama.com/US%20%20%20%20596326.html
You can get the 18-70, 70-300, and a D300 or a D90 and still stay within your budget.