Keep it cool - good airflow inside your computer and around your drives
is important.

Is there evidence to support this? My drives are hotter than usual because
they're decoupled from the case. 40 degrees or so is hot but the the
manufacturers' max temperatures are 55 and 65

A good aluminium case will act like a heatsink to a modest degree, however
if you have several drives mounted one atop the other, the heat from each
one rises up to the next. Without adequate cooling, the drives (particularly
those on top) will get toastier and that has the potential to shorten their
lives. Most case airflows run from the bottom front to the top back, pulling
in cool air near the ground, passing over the CPU and MB and out near the
top. Often this airflow doesn't serve HDs well (excepting cases where HDs
are mounted at the bottom). This problem can be exacerbated by using wide,
flat, non-aerodynamic IDE cables which block efficient airflow. You can swap
these for rounded cables or fold the existing cables in two twice over and
strap them in the folded shape, or you can switch to Serial ATA.

There are many arguments about what temps are acceptable. Usually there's a
trade-off between cooling noise and acceptable temps, but there are many
solid cases and low noise fans about. Personally I've found 40 degrees to be
quite acceptable for a HD temp (also for a CPU temp too), although the
ambient case temperature should be much lower!

Like much else, these things will *contribute* to a healthier PC and to a
longer component life.