I don't know how the D600 buffer is, but unless you have a 500mm or 600mm lens, I'd recommend staying with APS-C. My budget limits me to 400 f/5.6 as my longest lens, which means APS-C for me.
That said, you're going to want the D400. If your D300 is done for, you need something in the interim that will work as a D400 backup when you get it. It's your call whether upgrading to the D7100 sensor and compromising on buffer is the best option for you. It sounds like the focus issues of the D7000 have been addressed in the D7100. It doesn't have the ergonomics of the D300, but otherwise it seems to be a pretty good camera in most respects except buffer size for wildlife shooting.
I had a similar dilemma when my D200 died about a year ago, and selected a bargain D300s refurbished. It's been a great camera, but the D7100 wasn't an option when I upgraded, and today I would seriously consider it, assuming that it's only 6 months to 9 months until a serious new top-of-the line DX camera will be out there and I would have the best of both - a capable action camera, and a state-of-the-art sensor.
Based on my experience and reports from a friend who has both D300 and D300s, the D300s has some subtle but significant improvements in operation and handling, better AF, etc., and you may be pleasantly surprised by comparison with your D300 once you have a chance to use it.
|DSC_9643 by NOWHITELENS|
from Best Photo of the Week
|Thailand Sunrise by ozziebadger|
from Ships and Boats