With RAID 5 though you actually get more capacity in the OS. I use a small standalone RAID 5 box at work with 6 14GB HDDs in it. With it running at RAID 5 Windows sees it as one drive at ~67GB. If I just did RAID 0 or 1 (using just 2 drives) Windows would only see one drive at 14GB. The only drawbacks I see with RAID 5 are cost, and if in the invent 2 drives failed you're could be screwed.