I use mine to run a 54" Bravia HDTV as the extended desktop and use EyeTV and noticed numerous graphics errors lines, and failures to refresh the screen during overheating bouts. I observed that the upper left hand corner of the iMac has some sort of an air pocket that traps extreme heat from the graphics card there. (The rear-viewed 'right' of the top air exhaust slot).
This top left hot air pocket-trap seems to be due to negative cabinet air pressure interfering with natural heat convection on the left side; heat collects there but just can't get out! The fans are part of the problem and just don't help at all.
To verify this, I installed a small temperature probe in the back top left hand (from the front) side of the back air exhaust slot (right side viewed from rear) around 20 cm (9") from edge. The usual temperature reading there runs around 130ºF (54ºC!) and often (always during malfunction or addressing error graphics lockup-freeze) exceeds a peak-hold temperature of 140ºF (60ºC).
At first I mistakenly thought forcing cooler air into the bottom might reduce this but it does not. There is an air pocket there caused by the vacuum back-pressure of the internal fans, which are all trying to suck air in through limited air inlets, and blowing out the upper right side of the slot so heat just accumulates there and it just runs away
The correct and ideal simplest way to permanently fix your overheating 24" Imac (iMac6.1 iCore 2 Duo at least, and several other models) requires two fixes.
First download and install smcFanControl 2.2.2. Set it up to provide (still quiet) minimum fan speeds of 2200 rpm for the HD and 2800 rpm for the CPU, leave the Optical Drive fan at 600 rpm to minimize dust accumulation (or just tweak it up if/when you will be using it much)
Next obtain a 12 vDC enclosed squirrel cage fan like the San Ace B76 - you need not buy a new one nor this particular (best) model, but any enclosed suction-blower fan of this type (rather than an open blade straight exhaust) is ideal. It shouldn't cost much at a surplus shop. Get and hook it up to any common 9vDC/AC adapter (around 10 vDC) to run it cooler and more quietly than full 12v speeds, since it is now sucking heat instead of blowing cold air.
Tape it (with clear boxing tape) to the rear left corner of the back (3" from the side) so it sucks air out of the hottest point of the back slot and blows it away upwards.
Your iMac will now be fixed - the air slot temps will seldom ever exceed 106ºF (40ºC) again (even under heavy graphics use) and your graphics card will stop cooking.
Barring other serious dust or other heat sink assembly issues, your iStatNano display should now (roughly) read (degrees C): (at 26ºC Ambient room temperature)
- HDD 42º
- CPU 33º
- GPU 40º
- GPUD 46º
- GPUH 38º
- Amb 26º
- DVD 1 39º
[crarko adds: I haven't tested this one. Has anyone else needed to do something like this to prevent over-heating? If so, please post your experience in the comments section.]

