Not that MP is everything, but if you shoot DX on the D7000 (Only choice by the way) you get 16.1MP. If you shoot DX on the D600 (that is what you would need to do if you use DX lenses), you will only get 10.3MP or about a 36% reduction in MP.
Shooting DX on the D800 would yield 15.4MP which is virtually the same as the D7000.
I personally couldn't see a good justification for "upgrading" to the D600 unless I could also afford the FX lenses. If I could afford an assortment of FX lenses, then I should be able to also afford the D800.