No, it is not a supported config on any Dell controller (although you can tweak it out on the older controllers).  The SAS 6 is a low-end controller, capable ONLY of RAID 0 and 1 (not 10 or any other "nested" RAID level). "If I plugged six 1Tb disks in, can you estimate how much storage I could realize without resorting to JBOD and individual drive letters?" Without resorting to JBOD, the math is simple: Redundant:  2x1TB RAID 1 = 1TB (930GB after disk math)Non-redundant:  6x1TB RAID 0 = 6TB (5.6TB after disk math) Obviously, the redundant option does not "net" you a lot of space, and the non-redundant option is, well, not redundant. Mixing hardware and software RAID is not advisable when redundancy is partially dependent on one or the other, but you could stripe 3 on the controller, stripe 3 in the OS, then mirror hw R0 with sw R0 in the OS (2.8TB usable space), but then I think you over-complicate things for Windows and likely sacrifice too much performance - and possibly reliability.  You could do JBOD and let Windows manage them all in a software RAID 5 (4.6TB).  
I need to add storage to a PE 2900 with a SAS 6/iR integrated controller. Two drive bays have 1Tb disks raid 1 for the OS. I want fill the remaining six bays with disks, but am not sure how they can be configured. The controller only supports raid 0 and 1, and a maximum of two virtual disks. Is it possible to configure the six new disks using raid 1 - will it group the disks in three and three, then mirror those two groups?
SAS 6/iR disk configuration options