Upgrade from D80 or new lens

I'm going back and forth between upgrading from my D80 to the D5100.  However, I was wondering whether it would be more helpful to upgrade from a kit lens to somethinig like the Nikkor AF-S 16-85 mm VR.

So, what do think it is better to stick with the D80 and upgrade the lens or switch to the D5100?

Based on limited resources, I need to stay in the $600 US range.

All input is greatly appreciated.

Thanks!