I see this statement all the time. You have to reinstall Windows to implement RAID 0. It's simply not true. All you have to do is install the Intel Matrix Storage Manager, activate the Isastorv service instead of the Isastor service, build your RAID array, and clone your current hard disk drive to the RAID array. Thats it. Id did it again today with Server 2004R2 so it will also work for Windows 7.
And if you only buy the one drive? You could still do it by using an image, but that can be difficult if you have a 1TB drive 80% filled.
And if you have a AMD system...or you hdd are connected to 6Gb sata ports?