Not an entirely scientific test, but normally when I clean out the dust & crud from a machine, even works ones I don't keep track of running temps so don't know the knock on effect.
Anyway I always knew my single G4 MDD ran around 38-39+C, but the last few months this has suddenly jumped to around the 43-44+C mark, still well within tollerances so I've not got round to touching it, and it looked relatively clean inside. Today I'd got nothing better to do so I've just dropped the side panel down & cleaned the grills behind the nostrils which now had a film of dust over the holes. Not a huge amount, but the effect is obviously noticable. I'd left it running while cleaning, and so the temp knocked up another 0.5+, but closed up again within 5 mins, it had pulled the temperture below 40+ for the first time in months, and it is now happily sitting at a constant 38.8+, back where it used to be. So god knows how much of a temp difference it makes to iMacs that tend to get absolutely crudded up inside the dome!