Thanks for that comprehensive response, it confirms what I feared. Still even with a Q9550 processor the existing temperatures remain cool enough for my demands so I will leave well alone, although an upgrade to the front fan to improve the air flow might help, as might an additional fan at the back as I have replaced the hard drive with an SSD.
Looking to upgrade the cpu cooler on a Dell motherboard, part number MK 524, in an Optiplex 780 SFF but run into the problem that the screw holes for the stock cooler don't seem to match any coolers I can find. The existing holes are approx.3.5" apart in one direction and 4.5" in the other.  Anyone any suggestions? 
CPU cooler suitable for Optiplex 780 SFF