First and foremost, if you ever open your PS3, even after the warranty is out, Sony has the right to refuse to let you send it in for service, this doesn't apply to changing the hard drive on the old fat models.
Second, if you're fairly handy with electronics and you don't ever plan on sending your unit to Sony you can put in a manual fan controller so that your unit will never get hot again. This can prevent most YLODs or, if you've already had one and reflowed your mobo, it will keep it from happening again. While you're in there rooting around you may as well change your thermal paste on the CPU & GPU because the stuff from the factory is absolute junk.
Third, if you can remember hearing your PS3 fans running, but now you can't hear them and it doesn't seem to be blowing any air it's possible that it's stuck on low idle like mine was, which subsequently caused it to get hot and freeze constantly.
Fourth, never ever ever let your PS3 freeze or shut off during a firmware update, it will brick your unit, you will have to send it to Sony, a new hard drive won't fix the problem. If you're unit is freezing for some reason and you reformat the hard drive don't reinstall the firmware until AFTER you fix the freezing issue.
I've learned these things in the last week, all of them the hard way, I'm just passing on what I know, hopefully it will save somebody $150 and their PS3 a trip to Sony in a cardboard coffin.
In a related story I have a working 120gb PS3 slim, and an old backward compatible 80gb with 4 holes drilled in the case, a manual fan controller, new thermal paste, and screwed up internal flash memory that is currently operating in doorstop mode.