I Don’t need mirrored drives for my OS, theres nothing critical on it, its mainly a gaming box, I have 2 500GB storage drives in there already for data storage, the RAID I could care less about, Its holding only my OS and installed apps… If a drive goes down, oh well throw a new drive in and reinstall windows.
Forgot to say, one of the 320GB drives was DOA, so I am waiting for an RMA, running single drive right now.
My Vista Experience Ratings are 5.9(Max) straight across the board, EXCEPT HDD Transfer speed, which is a 5.6… Cant wait to get my drive back 
RAID 0 is bashed by most because it has NO redundancy, One drive failure in the RAID leads to complete data loss.
But for something where you are not worried about longterm data storage, and are looking for the fastest data transfers possible, RAID 0 is Ideal.
If you are backing up to the backup drive daily, and the only adverse effect of having the RAID crash is needing to replace the failed drive and reinstall windows, I would leave it just how it is for the fastest data access.