<p>Something else the OP wants to consider is this.<br>
Going from a 12 MP DX sensor to a 24MP FX sensor will not result in a loss of reach. So really, you don't lose anything but you gain superior high ISO capability. I found this out when I went from a D300s to a D610. I shoot birds with the 300mm f/4 from a hide about 3 meters from my bird table. Taking the doubling of MP on my 610, the birds fill roughly the same area of the frame as they did on my D300s and they crop, in post, to about the same pixel resolution, so you're not really losing anything. But now I get lovely noise free images at much higher ISO's than my D300s could provide, so I'm as happy as I could be.<br>
Obviously you would lose reach if you went from 12MP D90 to 12MP D700, or 24MP D7100 to 24MP D610, but not from 12MP D90 to 24MP D610 (or D750).<br>
Also, don't discount the D610, it's a great camera for the money.<br>
But I agree with others that a better lens would make more sense.</p>
