I have raid 0 for years, I had not one problem yet. Just think about it, how many time did you loose the data of your hard disk? Zero? That's the reliability of today hard drives.
You want absolute speed? 1 velociraptor is enough.
Don't have much money? Raid 0.
Why do you want raid 0 speed?
_to boot the os faster? You're loosing your time, the speed difference is negligible, maybe 10% more speed.
_to load game faster? Yes, your games loads quite fast ... from double click on the desktop icon to the game menu, to load the levels faster the best option is having a very fast CPU, like 3Ghz or more. It doesn't matters the number of cores the cpu has only the speed of the cores, the gigaherz'. A raid 0 will not load game levels faster if you don't have quite the cpu.
_Defragmenting the hd is faster on raid, coping files from one side to another is also faster.
I hope this serves you.
PD: If you want to raid buy 2 new HDs of the same model, this way you get all the speed a raid can give. 2 different model HD will not give you all the speed.
If you're gonna raid, buy 2 disks, and use your old disks for backup of your important files. I had never backup a thing, if I burn out some data to dvd is to free disk space only. Bye, good luck!