Swapping the 65W core 2 duo CPU with a 95W quad core puts quite some stress on the stock cooling system: the cpu runs almost at more than 70+C and the fan is very loud.
First step is to get a DELL CN-0JP911-41362 heatsink for OPTIPLEX GX745/GX755. They can be found for really chep on ebay, I got mine for 5 buck with shipping.
Second step is replacing the loud 66CFM Dell fan with a 30CFM Noctua NF-R8 1800pwm. It requires some soldering to adapt the fan to the proprietray Dell connector.
Third step is undervolting the CPU with Throttle stop: 1.1v is perfectly stable in this system.
With a room temperature at 27C:
I built a little prototype to see if I could get some improvement: basically an horizontal fan duct that goes inside the case. I used another Noctua NF-R8 picking up 5v on the second internal USB connector.
Tried to blow air in, and blow air out. It makes absolutely no noise.
When blowing "fresh" air in, the temperature gets worse by 3C.
When blowing hot air out, it does absolutely nothing.
Only with the fan at max speed did I see a 6°C improvement. But I can't stand the noise.
I eventually decided optimize airflow and noise by cutting the plastique blades in the front panel, and I put a big 12cm exhaust fan in the side panel. It's undervolted to keep it silent.
The results are pretty impressive: less noise, less heat: in a 26°C room, the CPU reached 48°C under load.