SD Cards don't have performance of an SSD drive. SD, like typical low-cost flash memory cards/drives are very fast for what they do, used mostly in the portable market - are not designed for the high-performance demands of a desktop computer. They'll wear out faster than SSD drives and not have the performance. Nevermind not even coming close to the SSD-PCIe drives ($1500+) which can transfer 1500~2000MB/s!
Lets do some math.
The SD-RAID0-SSD Device is about $150 (I don't know, lets guess)
Each 32GB SD-Card is $100 (Some are down to $80, but for math reasons, its easier)
So for 320GB, you'll need 10 cards = $1000 ($800 for the cheaper ones) + adapter = $950~1150.
Well, they don't make 320GB SSD Drives, but 256 and 512. 256 = $600, two of those would be $1200.... same price that OCZ is offering their 512GB drive.
Its not cost effective to use SD as a cheap SSD.