tl;dr: Dump the raid1 array, format and install one of the disks and you're good to go.
You mentioned you're not tech. savvy, so I am guessing that someone talked you into/convinced you to use RAID1 as a backup. RAID1 isn't a data backup solution, and it shouldn't be used as such. It's only good for minimizing system down-time due to hard drive issues--for something like a system that needs to be running literally all the time (server, etc.). What RAID1 does not do is any data integrity verification as it performs the mirroring of data. What that means is that if you have corrupted data on one disk, it will get copied to the other in a very "dumb" way. Meaning if you get something like a power surge that ends up corrupting some/all of your primary drive, all of those corrupted sectors will get mirrored to your mirror drive, and then you'll have two worthless disks and no way to retrieve your data. What you should do, if you want to upgrade to 64-bit is backup all of your data (this could be simply disconnecting the mirror drive since it should contain all of your data), formatting the primary drive and installing windows on the primary disk. Don't bother with the RAID array, and just use the other disk as a manual backup (external enclosure, etc.).