Before you install you must consider "mirroring" as it is the standard for production env databases. This would mean using Suns software or buying the Veritas software which is easier to use. This eliminates one of your biggest weaknesses of lossing a disk.

Then you have to choose how many disks will be mirrored. It is important to consider how you will "multiplex" the control and redo files- that means that they are written to 2 locations to protect you if the first one blows.

Basically the main install can be on:
/U00
The main tablespace *.dbf files can be on:
/u01
The backups and exports + multiplexed redos & controll
/u02

That means your talking about at least 6 SCSI disks as these 3 disks should be mirrored. People could argue for hours specifically where everthing should be put- just try to think logically about "if" this disk goes and has the control(or redo) --- where wil the other one be.

-Roger