I could be wrong, But in Raid0 the size is limited to the smaller one - but performance of iindividual drives are retained.
So thoughts on SSD Raid0
(1) You lose win 7 trim and must rely solely on CG.
(2) Raid0 will boost Seq read/writes
(3) Raid0 will NOT improve access time
(4) Raid0 does little to improve the small file random read/writes.
Myself I'd consider getting the NEW SSD as a Boot SSD, and converting your OLD SSD to a scratch disk and load the files that you want to work with.