I had a D2x, with a 17-55 and 70-200. I now have a D700, got a 24-70, and gave the D2x and DX lens to my daughter.
I think the D700 is balanced with the 24-70, and still feels balanced with an SB-900 on top for portraits. The 70-200 in fact balances better on a tripod with the D700 hanging off the back, than it did with the D2x.
You should consider some morning exercise routines with dumbells, and sit-ups to strengthen your back. The D700, 70-210, 24-70, and 14-24 in a Lowepro Slingbag with only a few extra goodies comes in over 18 pounds. I feel like I am carrying around a medium format kit.
But, then I see the pictures, and it is all worth it.