Conditionally chained proxy passes

Hi

I am in the process of moving from one server to another. Both servers
are
set up with NginX serving static files and proxying on the rest to
Apache.
What I would like to do is proxy everything from the old box to the new
one
using nginx.

However, I need the original IP address to check for throttling and log
file
stats. A simple setup gives me the IP address of the old box.

Any suggestions?