I cannot make sure of SSDs, but HDDs Raid 0 is actually worse than none. I tested this extensivly, and the truth is, if the HDDs are almost empty, raid 0 i a lot faster, but once you get it quite full, search times are way longer.
I read somewhere that raid 1 dosent have this problem (thou it does make you lose hdd space).
In SSDs, as far as i know, search times dont change untill its about 80% full, so its possible raid 0 will outperform regular ssds.
The 840 uses cheaper TLC NAND, I would go with the 830 as its has a proven track record. Overall the 840 is faster but only in benchmarks and you would not notice the real world difference. The 840 needs sometime to prove its realiability, 4 drives in raid vs 2? 4 drives will always be faster.