Dell Optiplex 745 SFF Reverse Cooling?

When I was engineering electronics, I generally mounted a fan to exhaust air from the chassis rather than pull air into the chassis.  There are reasons for both.  An exhaust fan will pull air into the chassis from any open port or leaky seam in the chassis.  For best results the chassis needs to be designed so that the openings are in a good position to cool things that produce heat; any seams in the chassis where the cover is fitted should be designed to seal so that the air only comes through the desired openings.

A fan that pulls air into the chassis can be designed so that the air cools things down and it isn't critical that the chassis be sealed to direct air flow.  One thing to remember with Dells, and perhaps other computers as well, is that there is often a temperature sensor located in the air that is entering the chassis.  This sensor is intended to monitor the air entering the chassis and is used to control the speed of the fans.  Warmer air means the fans need to increase speed.  If you place the sensor in the way of the air coming from the processor heat sink, the computer may become a bit noisier as the fan is driven to speed up by the warmer air.

This chassis has the fan located in a position to cool the processor heat sink first, sensible as that is the most critical heat producing item in the chassis.  Turning the fan around could result in the processor temperature going up if there is not a reasonable seal between the fan casing and the processor heat sink.  This results from air leaking past the heat sink directly to the fan.  You might want to check that if you decide to try turning the fan around.

So far as the rest of the chassis being "warmed" by the air from the processor heat sink, the temperature rise isn't usually that great and the air can still provide reasonable cooling for the rest of the chassis.  Don't forget that most of the other components produce little heat compared to the processor.  Regardless of what you decide, however, the computer will likely continue to operate just fine.  I don't believe the difference in chassis temperature between the two fan positions will be all that great.