Single HDD's wont go over 5.9 on windows scale. If your raid 2 of them in 0 you'll get 6.0. The only way to go higher is with an ssd. You should see a slight increase in boot up times, program start ups, etc etc. But dont expect anything huge. The benefit will be very slight and hardly noticeable.
Im not sure about the noise, it might be defective if its making unusual noises, but ive dont have experiences with the caviar hdd's so i cant tell you what theyre supposed to sound like.