you can raid with different size drives and different speeds, etc with most integrated raid controllers. Its just not considered optimal. in the case of the 640 and 500 you would not be able to use the 140 extra gigs on the 640 in any raid configuration except the JBOD (just a bunch of disks) which is not really a raid level from most points of view. Personally, if you are not too sure about raid then raid 0 is not for you. Its speed advantage in a home computer is questionable these days and it risks your data more than a single drive. But if you want to play, then play.
It's like going for a jog with a friend. It's not necessary for both of you to have exactly the same maximum speed to jog together, but whichever one of you can run faster won't be working to his full potential and you won't get to your destination as quickly as if you were both that fast. But you'll still get there faster than walking.