The following table is from the setup of a medium sized multipurpose server where I once worked. Aside from being a general Linux machine it will also be a network related server (DNS, mail, FTP, news, printers etc.) X server for various CAD programs, CD ROM burner and many other things. The files reside on 3 SCSI drives with a capacity of 600, 1000 and 1300 MB.
Some further speed could possibly be gained by splitting /usr/local
from the rest of the /usr system but we deemed the further added
complexity would not be worth it. With another couple of drives
this could be more worthwhile. In this setup drive sda is old and slow
and could just a well be replaced by an IDE drive. The other two drives
are both rather fast. Basically we split most of the load between these
two. To reduce dangers of imbalance in partition sizing we have decided
to keep /usr/bin and /usr/local/bin in one drive
and /usr/lib and /usr/local/lib on another separate drive
which also affords us some drive parallelizing.
Even more could be gained by using RAID but we felt that as a server we
needed more reliability than was then afforded by the md patch and
a dedicated RAID controller was out of our reach.