The query is as follows :
A typical server configs when using Oracle or any other type of DB is to install the OS + DB binaries on the internal disks of the relevant server e.g.
Disk 1 : OS + SW + DB binaries
Disk 2 : Mirror of disk 1 (used for resiliency)

Then one uses an external array that is typically connected via Fibre Channel Card from Server to Storage Array.
One then configures the storage array either in RAID 10 or RAID 5 (typically a trade off between READ/WRITE speed vs. STORAGE DURATION/LENGTH).

Now the question is if one had a server which has the capacity of having 6 x Internal Disks (e.g. 6 x 146 GB 15K rpm SAS) what is the trade off if:
(1) Disk 1 = OS + SW + DB binaries
(2) Disk 2 = Mirror Disk 1 (e.g. RAID 1 config)
(3) Disk 3 & 4 are then RAID 10 to Disks 5 & 6 , where Disk 3 & 4 hold the raw vols for the DB

Would it still be better to use a Fibre Channel Card to an External Storage Array to house the raw vols of the DB and why ?

If you need a lot of performance it would be a good idea since an external disk array usually has (hopefully) a lot of RAM to cache requests that the OS and internal hardware does not have to handle.
We usually have all RDBMs binaries and the DB's themselves on more or less powerful external arrays.
They also handle the RAID for you - you can also have them maybe set up redundant in 2 different locations just in case of a disaster. External storage makes you more flexible too but is also much more expensive.