In your case, putting all of your disks in one RAID volume will probably not help performance. Ideally, you should have multiple volumes that contain multiple disks in order to maximzie throughput. For example, with your 5 disks, you could have a 3 disk RAID 5 volume and a 2 disk RAID 1 volume. This way, you could spread your I/O over multiple spindles but still limit your I/O contention by placing some files on the RAID 5 volume and some other files on the RAID 1 volume.

IMHO, you need at least 12 disks over 2 controllers to have decent performance in an Oracle Database.