G5 imac (and i guess 1st G's of Intel's) - Cleaning - Overheating

I cleaned my iMac G5 inside in june of 07, when i exchanged ram.
Today the really bad 'nightmare', where it just -pop- dies in front of you, happened..
After this I tried restarting it, but got no reaction..
Then I waited (room temp. was around 30) for about 30 min; till I could feel it was cool (it basically heats just on the right side of the apple logo) and tried again. And it worked. So, logically, I turned off the computer, waited for it to cool down, opened it and cleaned it. While the computer was cooling, I called to my local mac store and asked the computer has an auto temp. shut off function, which it has, so that was what caused the 'death'..
The actual dirt that caused it to overheat was inbetween the grill in the bottom of the iMac and the casing for the power cord, and all the other casings in the bottom. Not a place you really can reach by just normal cleaning..
So, i have decided to make this cleaning a biannual event.

It leaves me with two questions;
What is the temp at which it automatically shuts off, and is there a way to monitor the temp?

Here's a good app that monitors temperature. You might consider visiting his site to obtain further info.
http://www.versiontracker.com/dyn/moreinfo/macosx/19994