As a fellow D700 shooter, I do not feel buying a spare D700 is the best investment. If you the NAS syndrome, you might want to consider spending the funds on better glass. Your profile doesn't show what you have.
DSLR technology keeps advancing.. less noise at higher ISO's, better focusing, better low light sensitivity, faster internal buss speed, etc. Unless you abuse the D700 or take 10 thousand pictures a year or more, you should have many, many years of use with the D700. If repairs are needed you have the D7K so you won't be without a camera. If the file size of the newer bodies is an issue, you can always shoot in crop (DX) mode. I understand that the D800 is a better 12mp DSLR than the D700.