What's the real benefit of RAID array's?

I just finished my first build and put one 300GB sata 1.5 hdd in as my system drive. Now I’m wondering if I should try to do a RAID 1 so I’ll always have a backup of my system hdd.

Do all hdd’s that go into a RAID array have to be the exact same model (speed, size, etc.). For example, could I put a 300GB sata 3.0 hdd in an array with the sata 1.5 I now have? Needs some guidance here folks.

Remember, I’m totally new at this. I’d appreciate your detailed input.

The size should be the same, speed won’t matter. Especially in this case where the primary drive will be “slower” than the backup anyway.