If you have the money, and you really felt you wanted to get into this and not limit yourself, I wouldn't get a D3, I'd get a D700 which is more or less the same except it's muchhh smaller and you'd actually take it some places with you.
The difficulty is you need to work out, do you want to do this quite seriously, or just play about taking fish photos about etc? If that's the case, get a $800 DSLR or something, you'll be happy. The problem is I know alot of people done that then wanted to upgrade really fast so you end up spending again. I think what zypher said is wise, spend once spend wise, but then I also agree with jcardona in that alot of people just want a cheap DSLR that will improve on their P and S.
I had a D90 then went onto the D700 for it's amazing low light performance, FPS and sensor. I also just drool over the fact all the buttons are on the camera and hardly ever find myself going into the menus. I am saving up for a 24-70 2.8 though over here thats about £1200!
it's easy to say that when you got the money to throw $5k into a camera setup that's just a hobby. if i had the money, i wouldnt be a tightwad 
About half the money I've spent on camera gear came from breeding african cichlids. 
$5k is only about half what I've spent, tbh. I did all the upgrades, starting with a D70s. That's why I always suggest people buy the best they can afford right away. There's guys out there who have gotten sweet deals from me on dx lenses. No more! lol
