Could be anything from a D40 to a D4. It all depends on what and how you shoot. Do you shoot sports needing alot of frames per second? Clean high ISO performance? Wildlife where the 1.5x crop factor of DX would be an advantage? Without knowing more its pretty tough to make a recommendation, though for general use, a D90 or D300(S) would be hard to beat. Even going back a generation to the D80 if you don't need the high iso performance of some of the other models.
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