Has anyone tried to compare those python implementations?

pypy
psyco
unladen swallow (is it dead?)
cpython

I am planning to squeeze something more from my server.
Setup:

Django 1.3
Python 2.7
Psycopg2 1.4
apache 2
mod_wsgi
and... Windows server

I am not a windows fanboy, but it has to be :{ There is some legacy code working on it.
One thing you should considerate is the C extensions. Different implementations require different extension ways. At present, the CTYPES may be the most common one.
So I recommend you take CPython, in case of possible C extensions.
django: on pypy, psyco, unladen swallow or cpython, which one is the fastest?