your cpu should not go past 95 to 100 c for prolonged time.
your hdd should not go past 60 c for long time periods
your gpu should not go past 95 to 100 c ( not sure about this )
So Basically,
1.Dont use handbreak to convert video (Runs my CPU around 105c for the duration of the encode with fans maxed out, ive fried a CPU doing a conversion of my Stargate DVDs to apple TV format in a que overnight) I now use a CPUthrottaling script that limits handbreak to 75% of one core, my encodes take 30% longer, but my CPU stays around 86c after 4 hrs + of encoding
2.Dont do anything that is HDD access intensive, install as much ram as you can and ask the drive to spin down as often as it can, better yet, run any HDD access intensive software of an SSD drive, or external HDD.
3.dont play games (GPU temp often goes over 100c if playing a modern 3D shooter or RPG)
and remember an increase in temp of one of the components will add to the temp of the others within that tiny confined space
Apple macs are desgined to sit right on the knife edge of the thermal limits, and are assembled by people who are effectvly treated like slaves in a labour camp, Apples gear looks pretty, costs lots, but that does not mean its better quality than a home built PC , in fact that home built PC will probably out last the apple by several decades of use. Apple doesnt care that running at the thermal limits all the time shortens life, they want you to buy a new one in three years when your applecare runs out anyway.
Newer macs are worse offenders of this than older macs, if something gets hot enough to fail, its should be easily user replaceable down the line, and if something is just a component that IS GOING TO FAIL anyway, even without the heat, it should be user replaceable, iMac HDD for instance !!!
I would point out im going to be buying an iMac shortly, despite this, because it fits my need of a low power, all in one, that runs Mac OSX. first thing ill be doing is a watercooling mod for the GPU and CPU.