The following configuration may help you to get best performance from the available disks:

Assumptions carefully considered:
1. Degree of Striping and Mirroring
2. Number of Available disks = 30
3. Data Ware Application with many users that require mostly unique scans on its tables or indexes
4. 2 Spare disks are also used for Striping
5. 2 Controllers each with 100MB bandwidth
6. 128K or 256K stripe would be better than 64K.

Volume Name,Number of disks, Striping, Mirroring, Used for
U01 2 No Yes Unix executables, Swap
U02 2 No Yes Oracle executables, REDO LOG Members A
U03 2 No Yes REDO LOG Members B
U04 5 Yes, Degree of Striping 4 No Index-1 Tablespace
U05 9 Yes, Degree of Striping 8 No Data-1 Tablespace
U06 5 Yes, Degree of Striping 4 No System, and Temporary Tablespaces
U07 5 Yes, Degree of Striping 4 No Rollback Tablespace
Total 30


Note 1:Reason for using 9 disks in u05:
As you said the minimum transfer rate from buffer to media is around 21 MB/Sec, you can very well go up to 10 disks (10 disks * 21 MB=210 MB) in one striping. More distribute the data, More Parallel I/O for Reading can be engaged.

Note 2:
If your application is Data Warehouse with few users that require many range/full table scans, then RAID 3 is better option than RAID 5.

Note 3:
If you need more disk space for your DATA, you can use U07 volume. Create Rollback Tablespace in U06.

Note 4:
If your database is in NOARCHIVELOG mode, then you do not need U03. Use those 2 disks as HOT SPARE. However, if the database is n ARCHIVE mode, then you must have 2nd members of REDO LOG.


Note 5:
Why do you need less degree of striping for Index than for DATA.
INDEXES require a lower degree of striping because the data in Indexes are always lesser than data in Tables.

Note 6:
Whatever be the RAID level, Never create INDEX Tablespace in DATA volume. This will definitely hurt performance.