You're right, that's a smokin deal for a pristine D700. But, if you are going to be using mostly DX lenses, I wouldn't buy either, although you would probably be happier with the D600 in DX crop mode solely because it has double the mega-pixels. Unil recently, I had never been a fan of the DX sensor, but if high ISO is your main objective, you might want to consider the very capable D7000 or even a D3200 which has the best sensor that Nikon has ever put in a DX camera to date. Your lenses would be less expensive and you will have solved your high ISO dilemma. Keep in mind that if you opt for the D3200 you need AF-S lenses to autofocus so your 50mm 1.8D won't autofocus. However, considering that the D3200 is $650.00 new with a very capable and sharp 18-55 kit lens you should have some extra money to replace your old 50 with the 50mm 1.8g or whatever else suits your fancy.