Before reading Jeff Atwood's post about Hard Drive Temperatures, I would have thought the the CPU is the most temperature sensitive component in a PC. That's what I focused on, when I was selecting parts for a new desktop at home. Jeff's article is full of details but he makes a solid point that is often overlooked - if your CPU or video card are fried, its not a huge deal. Sure its an inconvenience and a hassle to order new parts to replace them. If your hard drive fails - you lose data. And HDD are usually rated to work at up to 55C, 15 degrees cooler than most CPUs - and temperature seriously affects the reliability of the drive.
...increasing HDD temperature by 5°C has the same effect on reliability as switching from 10% to 100% HDD workload. Each one-degree drop of HDD temperature is equivalent to a 10% increase of HDD service life.
If you don't have good or recent backups, your looking at the situation my Dad thought he was in earlier this week. The hard drives on his PC were locking up windows, refusing to boot, and overall just causing random errors the whole time. Of course, he tried reinstalling windows (and surprised me by saying he's also leaving a partition free to install Ubuntu) but the problems wouldn't go away. Using an external usb connection, he was relieved to confirm that the drives were actually ok and the problem is more likely a bad ATA controller on his motherboard. A potential disaster averted.
What can you do to keep your Hard Drive cool? Jeff points at a small window applet that can monitor it for you. If you don't have a fan on the front of your case, you can pick one up and install it fairly easily if you have a plain vanilla case. Hard drive coolers are also available but a little more involved to install. I put one into my computer and found it didn't add any perceptible noise when the computer is running.