First, I think the D70s will be tougher and tougher to find. In addition, the D80 is basically a d50 body with alot of d200 features packed in it, and only slightly more expensive. For me, that makes the choice between the D50 and D80. All of your current lenses will work and cover a decent shooting range. Unless you need the extra features of the D80, I would probably just get the D50. Aside from a bit less control, the D50 outdoes the D70 (the d50 by the way is newer than the D70)..I probably wouldn't consider the D70 at all.
One other issue with the D80...If you need the camera now, it won't be available until Sept. 1st.