Paul is absolutly correct.  The number of disk is an issue along with
the number of drives on a SCSI channel.  Disc drives are getting bigger
and cheaper, not faster.  A disk will handle upto 30 I/Os per second
(disk arrays about 40 I/Os per second).  Based on how the data is spread
around on the disk it may reach 100 percent utilization around 15 I/Os
per second.  So if you have some idea what your I/O requirements are you
can get an idea of the impact of fewer disk drives will have on your
system.

The other thing to keep in mind is never mix disk drives with different
capacities in the same volume set.  MPE will balance the disk space
based on a disk drives free space percentage.  As files are built they
will tend to move to the larger drives and thereby reduce your I/O rate
as everything waits on the larger drives.

I would add one caution with the statement about adding memory.  As a
general rule I agree that adding memory is a good thing, but the old
addage of "too much of a good thing is bad" is true here as well.  Based
on how your system is used there is a point that adding memory will slow
interactive user performance.  We found this when doing performance
testing at HP's Capacity Planning Center.  It seems the more memory that
batch processing has to place with the more it dominates the system and
the priority of the user's versus batch has very little to do with who
gets cpu time.  It become a matter of who has resource locked (in use)
when a user needs that resource.

I can not give you any idea how much memory is too much.  It all depends
on your system utilization.  But when you hit that magic spot you will
know what it is.

Kenneth.