How big is your database, and how much memory does your database server have
for use in bufferpools. If your bufferpool hit ratio is high, it won't make
any difference.

You need to start with the performance guides and build your
understanding of the pro's and con's of each storage mechanism. Once you
have this understanding, you'll need to apply it to your expected load
and then verify it against the actual load placed on the disk subsystem.

Separate disks isolate physical arm movement of data and indexes. Raid
cannot do this because the array is (usually) treated as a single disk.
The simplest isolation; a separate single drive for indexes and data
will allow twice the number of arm movements per second than placing
both components of the data base on a single drive.

Raid-5 type arrays need to be examined very very carefully. The normal
write procedure for this mechanism requires reading data from all of the
disks in the array and always writing data to a minimum of two disks.
When the database manager is in a "wait for write operation to complete"
state, the potential delays for the multiple reads and writes can be
significant. Hardware is available for raid arrays that may alleviate
this problem.

Phil Sherman

Perhaps you should also read some of the articles from BAARF:

http://www.miracleas.com/BAARF/BAARF2.html

Jan M. Nelken