D600 with FX lenses?

I shoot a few wedding as year on a very aged D300 and always seem to be struggling for light. I am thinking of adding a D7000 or D600. The body only D600 is a big stretch cash wise and is double the price of the D7000. Getting a D600 will mean that I will be limited to my FX lenses and a 50mm 1.8D.

Anyone any experience of the image quality (esp high ISO low light) of the D600 with FX lenses VS D7000 with same lens?