I upgraded from a D40 to a D80 right before the 90 came out and it was well worth it.
My reason for making that small step was the higher sensor. focus points. fps and size and weight.
D40 i find is to small.
Don't buy a D200 it's like 4 years old probably more. and long discontinued. that being said you can get one cheep BUT it's old news. and why spend the money on the D300 to get the CMOS sensor when...it's in the D90 for cheeper and a better camera.
Fyi D90 is the exact size of the D80.