With 5 disks you have IMHO three choices:
1. Raid3, possibly with 1 hot-spare. It will give you 4 TB net storage without hot-spare or 3 TB with hot-spare.
2. Raid5, possibly with 1 hot-spare. It will give you 4 TB net storage without hot-spare or 3 TB with hot-spare.
3. Raid6. It will give you 3 TB net storage.
Personally I would choose option one. It depends on your backup process whether a hot-spare is desirable or not. There is one potential risk with your approach and that is the WD disks. If they are the Caviar Blacks you may encounter TLER difficulties, if they are RE4's no problem.
It may be worth to try what gives you the best performance:
a. 2 Raptors in raid0 on the mobo, or
b. 2 Raptors in raid0 on the Areca as a separate volume set, next to the raid3/5/6 array.
Thanks for the speedy feedback Harm - I appreciate it.
I was thinking RAID level 3 as well.
Of course, it's always something! I purchased the Caviar Blacks by mistake - which are non-TLER. I will work with EggHead to return the ones I purchased and replace them with RE4 versions as I'm not thrilled about the possibility of the controller declaring the volume/disks degraded unnecessarily and although I have the DOS utility WDTLER where one is supposed to be able to enable/disable TLER on WD drives - I suspect WD is way beyond that now anyway with current builds.
I agree with you about just testing the performance of the options for the raptors - on the mobo and then on the controller. When I benchmark them I'll post the results in case others are curious.
Thanks again....off to EggHead!