I popped a fan on top of my iMac g3/400 for 45C days - they have a thermal cutout that popped in from time to time, and it'd just turn off to protect itself from overheating. The fan solved it all - it's a 15cm ish one, and I made a cardboard shroud to tape onto the top of the mac. it sucks air out of the top of the iMac, and so pulls cool air in through the bottom.

http://www.danamania.com/tmp/imac_hat.jpg

Made such a difference!

(I put a layer of tape around the edge too, and taped more tape facing the sticky side too, so it'd seal a little better)

Dana