yeah backup will be recommended. i have backup for the data i don't want to loose.
but you only have 128gb that can read/write in those high speeds.
(less actually because when the drive is NTFS/fat formatted you loose a certain among of storage. you will have i believe only 115-120GB)
thinking about that: it ain't that much space that can be used at that speed. bootup and windows will be very noticeable faster.
but you will eventually be running out of space and you will start to install more and more on the Data drive.
SSD's are expensive in Price per gigabyte.
so i want RAID0 because it is affordable with a high capacity and you can see the difference when opening files compared to a single drive.
here is some benchmarks that shows single vs RAID0 AND
vs SSD. clearly the SSD takes the cake, but 2'nd place goes to RAID0
SSD VS HDD RAID -0 VS Single HDD Comparison - [H]ard|Forum
but again if you don't need that extra speed and want to save some money, go with a single drive.
I would just have gotten RAID0, but you don't have to.