depends what mode you use - many people use mode 0 which basically combines 2 discs into 1 but means you have 2 heads writing/reading so it performs faster. most other modes are corp server orianted and are used for redundancy on important data
Most of what I have read is that there is very little if any performance difference at the home level. Now, once you get into SCSI raid where you may have 20-30 drives in Raid 0 it makes a real difference.
Okay... say your installing a game.... Raid will take the program and strip it into 2 and right half on 1 disk and half on another making it preform twice as fast... Then when u pull a program off your hardrive into memory... it will take the two and fuse them together and put them into memory... Thus again... making it 2 times faster.... The less time the hardrive has to read or write... the faster your programs will execute thus a faster PC (i guess)