Depends, what's your I/O distribution? On a system with only 6 disks, does SYSTEM, RBS and Archive logs warrent a dedicated device?

I would look at my I/O patterns. If you have really hot data files, I would spread them out on multiple devices. If you are archiving once every 2 hours, I would put other data files on that device as well. With 6 disks, you want all of them to carry approximately 16% of the I/O burden.