Jeff,
What are you using the 3 drive array for? Is it just for backup... as in a Time Machine destination? If so, then I think you are wasting a lot of speed, since that is not a very intense operation.
It seems that you want the single drive to use a a "working drive" to keep your current projects. Depending on what you are doing, you may want that on the array, since you will get much better real time performance. For me... putting my Aperture 3 library on a Raid 5 array (4 drives) made A3 feel much quicker.
I decided that I am going to give RAID 10 a try. I was originally thinking I would stay with RAID 5, but I do not need all of the space, and RAID 10 is even faster. Even though my data is double backed up (TM locally, and Crashplan+ cloud)... I still prefer to use RAID redundancy... so I will not use RAID 0 even though it is the absolute fastest.
For backup, I have been using Time Capsules on my home network. I might switch to a Thunderbolt direct attached 3TB or 4TB drive for the iMac. I have a 3TB Seagate GoFlex now and I have a TB interface for it. I have not seen the 4TB models yet, but they are probably imminent. They are inexpensive and plenty fast for backup. This whole project is waiting for the release of the '12 iMac. My current iMac (Nov '09) is not TB equipped. Playing with my small stash of TB peripherals with my '11 MBA makes me conclude that TB is an absolute bargain (contrary to popular belief on this forum). This type of performance was previously only available in enterprise class systems at much higher prices. On the consumer side... TB is MUCH faster than USB 3.0 or FW.
I predict that you will enjoy your Pegasus. Good luck with it!
/Jim