there's a stark price difference in the two. By all means go with the D700 if you have the money, but coming from the D60 I think the D90 will suit your needs. That is in less you're going to a professional level and need the features of the D700.
what is you intended use for the camera? what is your budget? what is your experience level? What do you feel is limiting you in the D60 that you feel the need to upgrade? as jdeg pointed out the there is big difference between the d90 and the d700