Hmm... depends which RAID you go for.
I'm guessing your planning on going with RAID 0, which means it'll stripe data across two drives, to increase loading and reading speed.
It should boost, but I don't think you'd notice a mjaor improvement. I wouldn't personally do this for the fact if one drive goes, you've lost what was on both, since it needs both drives to read from.
RAID 1 is a nice one though, which mirrors, so write speeds are slower, but read speeds are slightly faster, and it means that if you lose one drive, you always have a backup, allowing you to buy a new one, and replace, and sync.