I'll do my best
Without professing to know much beyond RAID 0 and 1, I think I can clarify somewhat.
RAID type 0 is also known as striping. It basically interleaves the format of the hard drives so that sequential pieces of data are written alternately to each of the drives in the RAID. The only reason to use this setup is to eliminate the bottleneck that occurs when all of that data gets in one line to be written to the drive. It offers performance enhancements, but no redundancy...so if any of the drives in your array fails, you will lose all data.
RAID 1 essentially employs a built in redundant drive. All data is written, and then rewritten to the second drive. Since there are two drives with identical data, they can be read simultaneously. So, it helps some when you are reading, but doesn't offer any extra performance when writing. But, since everything is at least once redundant, it is more fail-safe.
I'm almost certain that you would have to reformat the drive you currently own for a RAID 0, because the drives have to be paired and interleaved. I don't know if having drives of the same make and model is necessary, but because the purpose of any RAID is to fine tune performance, and/or fault tolerance, it seems like having matching drives would be wise.
Two last things.
1) This might help:
http://www.acnc.com/04_01_00.html
2) I think from what I have read, that the magnitude of bottlenecking that has to happen before a RAID 0 would provide meaningful performance enhancements is hard to acheive in day to day life. You might be wasting time and effort to eek out a slight increase in speed. And if you're shooting for RAID 1, an external drive for backups might be easier.