I'm sure what I would do (D300s), and as a person who uses both DX and FX, I can tell you what I wouldn't do.
I would not get the D600 until I was fully aware of what it was going to cost me in glass. I would not try to run DX glass on the D600, even though it is possible. The viewfinder experience is just not very good with DX glass and especially under stage lighting conditions. I would not buy a D600 and then be dollar constrained and resort to consumer lenses with restricted aperture. You are already shooting at f/2.8. I'm make sure that still was part of the overall plan. Your longest f/2.8 "reach" with the D600 is 200mm with the lenses given. You could compromise some with a TC14EI and that will give you some of the DX look you currently have, but at f/4.
Anyway just my two cents worth. My philosophy is to replace a broken camera with one that suits me for the present and plan my "upgrades" carefully without trying to solve too many problems at once. I'd not reach back to another D200 because of the technology gap. I've had and loved a D200, but IMO it's time to move on. Hence the D300s, barring a full FX plan and glass to match.
Roger It's still, ISO, aperture and shutter-speed, right?