most machines will start to behave erratically. I.e. crashing randomly, locking up. Generally behaving like they have Windows installed, only more so.
Generally, the other side effect of heat is to accelerate the aging of the parts of your computer, specifically things like CPUs and hard drives wear out much faster when running hotter. It's one of the main environmental factors engineers use to determine the mtbf of components. They up the temp to different settings like 90C, 100C, 120C, 140C, and so on, figuring out the average life at those higher temps, then use non-linear regression to determine how long they'll last at 70C or below.
I was just joshing about needing 8 fans, but I'd have at least the PS fan, one entrance fan on the front blowing in, and one more on the back blowing out. then, you can buy a cheap temperature probe to check inside for hot spots when it's running.
There's also some nice hard drive fan systems that blow in on the drives through the 5.25 slots and house a pair of 1.25" fans blowing right onto the drive and into the case.
