smartctl -a /dev/hda | grep -i temp
This will return the drive temperature; you can create your own script around it to do whatever you want to.
While desktop drives typically run in the mid 30's, laptop drives typically run in the mid 40's.
If you have a desktop, you can replace your IDE ribbon cables with round cables, vacuum your case (especially the vents) regularly, and even add an additional fan to cool the drives. It's really a question of how much you want to keep your drives cool. If they are not generating errors and the temperature doesn't exceed 55, I wouldn't worry about it.