@Outlander: It's called Intel Rapid Storage Technology now. They changed the name a couple of years back.
I'm pretty sure Tepid is right. once you have a raid 0 array as your system disk, you have to start over again. Maybe what you want is to set up a raid 1 array, where an automatic copy of your hard drive is made. Read up on raid for your machine and in general. Wiki is pretty good for this.
Tepid's remark about similar sized disk is right on. Also beware of upper limits to raid array sizes. I think 2TB is about the biggest disk (or array) windows can handle. i certainly defer to the more experienced here though (Tepid?).
I am currently having problems with Intel RST, even though I am using all Intel hardware and chipsets and brand new hard drives. Even if you choose to use a raid array of some type, it is not necessary to install IRST. You should be seeing a setup option for raid arrays when you boot up your machine (if ACHI has been enabled). I strongly suggest you not install IRST, it will slow down your machine, and you cannot get rid of the drivers once installed without reinstalling the system.