Others have shown you the way, so I'll just add some info:
RAID-1 will cost some performance against a single drive setup and significantly more against RAID-0. Whereas 0+1 will have about the same perf as 0 alone. Just to be sure you understood.
. If it was me, I'd make sure I kept good backups (perhaps to a third IDE drive on one of the normal channels in a removable drive tray - or to a DVD or CD burner, worst to worst) and save the nickels until I had enough for another pair to do 0+1.
. Even then you still have to do backups - RAID-1 doesn't replace backing up if the data is important and/or irreplacable. It's just intended to help minimize down time. It is not unimaginable that certain types of system errors could corrupt/take down all 4 drives at once.
:Q
.bh.