When you get two hard drives on Raid 1 you only have half of the total space offered by the two.
In Raid 1, your computer will write files two times (once on both hard drives) for some extra proctection, if something happens on your "main" drive the other one is there with the same stuff.
In Raid 0, the computer will write half of a file on a drive and the other half on the second drive, which means your computer can write twice as fast and read twice as fast as well. Though if something happens to just one hard drive, neither of them can work properly.
This information probably isn't 100% accurate <img src="/ubbthreads/images/graemlins/smile.gif" alt="" /> but that's my understanding of how Raid 0/1 works.
