Keep whichever lens is best suited to your style of photography.
Personally if I was sticking with the D300, I'd sell the 24-70 and use it to fund a tele. If you're definitely getting the D700, sell the 17-55 and use the money to buy a longer tele than the first option.
Or if you find 24mm is wide enough on your D300, or have a 10-20 or similar, you might want to keep the 24-70 and D300, and sell the 17-55. It completely depends on what you shoot and how you shoot it!