As someone who's used all of those Nikons I'd have to say D90 if you can stretch to the budget for a second hand one. D90 blows D80 and D70 out of the water.
Another vote for the D200. if you like using older primes its a great camera I took some cracking dusk shots using s Nikon Series E 50mm lens which I bought s/h for £15. Currently using a 24mm Sigma 2.8 Ultra Wide which I bought for about £30.
A great solid camera - I luv it
Pete
For hands on - check the second hand camera stores like London Camera Excehange. None near bham tho.
If you are in London (or Reading definately has) loads of cameras... worth visiting if you can.
Derby is around 1hrs drive... nice people there.
Got my D200 !