RAID 0 is stripping, and it combines 2 hard drives of the same size, to be recognizable as one. It boost performance, because the info is being written and read, from two hard drives at once, instead of one hard drive. Downside is, if one fails, the data on the other hard drive is lost.
RAID 1 is known as mirroring, two hard drives. Or cloning in simple words. This is for security, so if one goes down, you have the other.
You can't RAID 0 your future 250GB HD to your 80GB, because they have to be the same size.