If the two drives you are configuring in RAID 0 will contain your OS & MBR you are going to have to destroy all data on the first HDD you are currently using. You'll have to start from scratch.
Shut down PC.
Install new HDD on proper SATA/RAID controller
Enter Dos RAID set-up utility. Which I believe is fasttrack. You'll be prompted to do so. Then you'll need to select Create. It will specifically tell you that proceeding will destroy all data on both disks. Go ahead and do it. Once the utility has finished, you'll then have (2) disks seen as one in windows.
But, now you'll have to install windows on the RAID 0 array. Put in the WIN setup disk and Install.