Honestly, if $ is an issue, Now might just be the time to grab a used D700. The grip and batteries from your D300 will work with it and the prices are really going down with everyone upgrading. There is a listing on our local Craigslist for a decent looking D700 for $1400 US. That's a pretty sweet deal, and I suspect it's not the lowest it will go. Yes, it's an older design/sensor, but what a camera it is!
Any of Nikon's FX cameras will make you smile, though. (assuming there's no major screwup with the D600...)