hi Dave.

I wouldn't recommend RAID 0+1 for your implementation. A simple RAID5 setup
will do just fine. The performance increase in using RAID 0+1 will be
negligible. The fault tolerance is not an improvement over RAID5. It's far
more expensive to implement.

I implement Exchange servers in environments similar to yours as:

mirror OS (controller 0)
seperate mirror Logs (controller 0)
RAID5 stores (controller 1) minimum four drives, preferably 5 or 6 just to
spread the operations across more spindles.
either a global hot spare and/or a good support contract (I use Gold support
4x7x24 with Dell)

That's my ideal config anyways. If money is short, combine the OS and logs,
but still keep the stores on their own RAID5.  A PE4600 with a split
backplane and drive cage will allow that config. You might have to go to a
PE2600 with a PV220 to do the config above.