You need to specify what kind of RAID.
Im guessing your talking about RAID 0, if so, you should get a decent increase in both read and write speed, but at the expense of the data protection that other kinds of RAID have.
RAID 0 can theoretically double speeds, obviously nothing ever actually works out so great. (2 drives = twice as fast, 4 drives = 4 times faster etc).
WD Raptors are a good alternative to RAID but expensive for the capacity.