I'm building a new system and wanted some input on the way I was going to set up my hd/ssd, I'll be using a OCZ Vertex 3 SSD (120g) for my OS + few games and three 3tb HD's for storage (movies/tv-shows)... I was thinking of putting the 3 HD's in raid 5, not sure if its worth it? I'll end up losing one of the HD's for backup right? Would it be easier just to grab a 6tb external HD for backup instead and stick with two non-raided 3tb drives?
Putting 3 3TB HDDs in RAID-5 will net you a total of 6TB of usable storage, since one drive is the parity drive in the array. This means you can lose up to one drive (either the parity drive or either of the data drives) and be able to hold on to your data. RAID-5 needs at least three drives to be setup, and as long as they're more high performance drives, it should last you a long time. Try to stay away from drives labeled as Eco and Green and drives that spin less than 7200RPM.
Remember also that RAID is not a backup; it's simply a way of protecting the data you're working with regularly. You'll still want to backup your data regularly to make sure it's safe. I would suggest the RAID array as it will increase transfer speeds to and from the drives (not as good as RAID-0 but still better than individual drives).