I have a Dell Optiplex 755. Replaced the old E4500 with a new CPU - the E8400. Runs much faster (Super PI - about 30 - 35% better).
HOWEVER, now the fan is stuck at super speed - sounds like a freight train. Dell says there are not any settings to slow it down. From what I can tell, the E8400 is supposed to run cooler than the older CPU.
Checked BIOS - and there are no settings affecting the fan speed.