I can't give you advice on a D600 or a D800 except to tell you I am not buying either as I don't consider them to be a true replacement for my D700. I did not go the D300 route. I started with a D200 which I really liked until I started shooting i situations where it just could not keep up. So when the D700 came out I bought one 4 years ago now and have used it for wildlife, portraits, scenic, action, macro, etc. If you plan to shoot sports from a stadium, I think you should keep your D300 or be prepared to either buy a longer lens or use TC's. My longest lens is an AFS 300m f/4 but I use TC's with it. My gallery is full of 4 years of images of all kinds. Others certainly have taken more spectacular images with theirs, but I am mostly a hand held shooter in low light, but sometimes use a tripod. So if you don't need video-I don't and you are not drawn to the D600 or D800, the D700 is hard to beat. ou can share accessories with your D300. It is rugged, does not weigh as much as a D4 and has so many customization features, it will make your head spin.

Bob Baldassano
My Nikonians Gallery

"Nikonians membership - My most important photographic investment, after the
camera"

Retirement is a gift of time - Don't waste it!