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.