I wouldn't worry about the buffer on the D7100 unless you are shooting sports, or very long bursts in raw. Frankly, I'd stay with DX if I were you. The D7100 is 24MP DX, and 16MP in DX 1.3 Crop mode. That's about the best reach available in any camera, unless you are using $10,000 plus lenses. For things far away, I see no advantage to the D600 over the D7100, and a lot of things the D7100 does better (like a newer and superior-in-every-way AF system to the D600). Now, if you were shooting macro or straight landscapes, or portraits, then full frame has it's advantages, and the 39 point AF system is more than adequate for things that stand still. But, the D600 is going to require FX lenses.
That said, I don't think the D600 and the D800 are in the same class. The D800 at 36MP is in a class by itself, right now. It's not for everybody, and if I were buying FX, I'd get the D600 instead, because I'm not good enough to utilize those extra MP's, especially at $1,000 more. That said, there are still things a skilled photographer can do with 36MP that he probably can't do with 24MP.
I remember when my friend bought a Porsche and was bragging that it would go 175 MPH. I said, "How will you know?"