I think the D600 is a great bet - as the elusive D400 is not certain by any means (they've been talking about it coming out for years).
And compared to your D300, you'll be seeing a huge improvement in dynamic range, image quality and size, color rendering, and high ISO IQ. It's also lighter.
On the downside, you'll lose a few focus points and the max shutter speed is 1/4000s, which is really not that big of a deal in most shooting situations.
You can always sell the D600 if and when the D400 comes out, although I wouldn't hold my breath on that one.
I think the D800 is highly capable too, although the file sizes can be unwieldy for some - and although I wouldn't go so far as saying it's a tripod camera, I think the D600 would be more forgiving for travel photography. Others may argue this point, but having experienced the D800 myself, it does require strict technique (and good glass) to bring out the best in your jumbo-sized 36MP images.