Use the SSDs are separate drives C & D.
The Read/Write performance of a RAID-0 array is based upon the speed of the slowest drive in the array. So your array's Read/Write performance will be based upon the speed of your SATA 2 (3Gb/s) SSD.
The total capacity of a RAID-0 array is based upon the drive in the array with the smallest capacity. So if you RAID-0 a 120GB drive and a 240GB drive then your array's total capacity (before formatting) will be 240GB (120 x 2). So you would be wasting 120Gb of storage.