You want a camera with low noise and the ability to capture unprocessed 12-14 bit RAW images (this is a little tricky apparently on the Nikons). Most amateurastronomers who use DSLR cameras have Canon or Nikon consumer/prosumer cameras. I use a Canon 400D but wouldn't buy one now since the new 450D has a larger 14 bit color space, live view update (very useful for finding the right focus) and apparently has lower noise. Other important features is mirror lockup and gentle shutter action (a bit of a problem with Canon).
If you are interested in shooting emission nebulae be aware that the IR cutoff filters in all DSLR cameras also remove 75% of the light from the H-alpha emission line. This filter can be removed or replaced with a narrower filter but this is a risky and/or costly operation that usually voids the guarantee and makes it trickier to use the camera for regular photography. Some companies sell pre-modified cameras or perform modifications but that is probably outside you stated budget.
I haven't tried light pollution filters but I know several people that shoot surprisingly good images from balconies in central Stockholm. Again such narrowband filters cost a bit of money.
If you you want take images of DSOs you need a stable equatorial mount preferably a motor driven or GoTo mount. When it comes to telescopes there a more knowledgable people around but a refractor or Newton of around 1 meter focal length is a common starter scope.
I have found
http://www.ayton.id.au/gary/Science/...toDigital1.htm to be a good resource.
I'm pretty much a beginner but I have attached a image I took of the double cluster low over the horison above Uppsala, Sweden (in a somewhat lit area during the full moon). It's a 3.5 min total exposure using a 20 cm Newton, no filters and an unmodified Canon 400D. As you can see I haven't so far) been able to completly remove gradient in the image.
Cheers
/Patrik