WD recommends its RE (they listed as Enterprise drives on WD's website) series HDDs for RAID, google TLER. You would probably be better-off using one for your system and the other for your frequent backups of the first HDD.
Oh. This sucks. I wish I had known about the TLER before I bought 2 720GB drives! If I dump the Raid array (BTW, just how would I do that???) and use one for backup - would this have an effect on system performanace? The other drive is still internal - is that OK to backup to an extrenal drive? Any other suggestions - I can't exactly afford right now to go purchase 2 new Hard Drives..