Measuring disk I/O

[Nerius Landys]

A friend and I are working on a small video-game related project as a
hobby.  We're running several scripts 24/7 that make lots of calls to
a MySQL database.  The mysql server process shows an average CPU use
of 1% (reported by top) and it never goes above about 2%  The tables
it's hitting are myisam tables.  I'm a little bit worried that the
mysql process is using a lot of disk access.  I don't know too much
about hard disks but my feeling is that too much disk use could slow
the machine down or cause a premature hard disk failure.  WD Raptor
model.

I don't know if my concerns are well-founded, but I would like to
measure impact on the hard disk somehow.  I don't know how to see disk
I/O.  I do know how to use top.  How do I measure disk I/O?  Any other
thoughts?