Is (1) 1TB Caviar Black 6Gbs better (2)150GB Velociraptors 3Gbs RAID 0

the velociraptors are faster than the caviar black, but they are only as fast as the first gen SSDs the current SSDs are like 5 times faster.
your better off just getting a new SSD for speed and better off with the hard drive for storage.

Yes, because 1TB > 300GBs. 300GBs won't hold a lot of stuff these days. Not at 20+GBs per game.