Overclex.com did a comparative test on Push, Pull, and Push-Pull configurations (unfortunately not a Pull-Pull), with a BIX. They found that a Push-Pull has negligable advantage over Pull, and that Pull worked better than Push. But we're talking, like, one degree C or so difference between various setups, so I figure efforts to optimise cooling are better invested somewhere else (e.g. ditch the double fan, concentrate on getting good access to cold outside air).