I am upgrading to the D700 from the D300. I currently have a Tamron 70-200 which is good in normal light but in low light on the D300 the focus speed is slow to never locking. I was curious if it is any better on the D700 or do I need to mortgage my house and get the Nikon 70-200 as well.

I cant answer your question, about AF on your D300, however i purchased that lens new for my EOS bodies. The AF was terrible inside my home at noon! hunting, not locking on. In its first week, it had some type electrical short, and jamed both my EOS bodies. I got a bad one. I doubt if an new body will help you. I would take your D300 and lens to a camera shop and try it on a D700.
Good Luck
Harry