"... Do I need Enterprise Class Harddrives in a Non Mission Critical Server ? " ==> By definition, if it's not "mission critical" then you CAN take some risk IF you're willing to.
However, Enterprise class drives have several features that are much better for RAID systems than a typical desktop-class drive:
(a) They are generally optimized for RAID environments, so you won't get drive dropouts while a drive is doing a reset to retry a read operation. The WD RE3 and RE4 series or the Seagate ES series drives are both excellent for this.
(b) The Enterprise class drives have higher vibration tolerances than the desktop series. In high-transaction environments (such as database servers) where you're doing a high IOPS rate, this can be very important.
(c) The Enterprise class drives have 1/10th the unrecoverable read error rate of the typical desktop drives. For example, both the WD RE3 & RE4 series and the Seagate ES series have unrecoverable bit error rates of < 1 in 10^15 bits. The desktop class drives -- even the excellent Caviar Black series (my favorite desktop drive) have error rates of < 1 in 10^14.
Since you've got a simple 2-drive RAID-1 configuration and are (apparently) well backed-up, you could reasonably use a pair of Caviar Blacks ... but personally I'd spend the money for the RE3 or RE4 drives. Contrary to what was said above, these drives are absolutely designed for 24/7 operation -- but it IS true that the high-end SAS and FC drives have even better bit error rates (e.g. the Cheetah SAS drives are rated at < 1 in 10^16). But for a simple 2-drive RAID-1 with non-mission-critical data the cost to move to SAS or FC is not, in my opinion, worth the difference. But I would go for an RE3, RE4, or ES series drive.
