Well, disk arrays can be built as fancy as your wallet is deep.
For a home media server or small business server, though, I can't understand why you wouldn't just use a SATA RAID. With a decent SATA RAID controller, and decent SATA drives, you'll get good enough performance, much easier part availability, and much much MUCH larger and cheaper drives.
Any decent SATA RAID card will let you configure RAID 0/1/5/10, same as a SCSI RAID card. And either way you'll easily have enough performance to stream a couple media clients HD content, while still recording one or two HD channels, while your torrents seed away in the background...
Hell, I do this with a simple on-board SATA RAID controller with two 7200 RPM (32MB cache) Seagate SATA drives in RAID 0.
And, as stated, if you need small but VERY fast, Solid State Disk (essentially a hardware RAM drive) is the way to go. It's going to be the fastest access available, and you can put your system partition and/or swap on there, and watch the performance gains.
SAS (Serial Attached SCSI) can be great in the datacenter, but the law of diminishing returns applies. In other words, you're paying a LOT for that last little bit of speed that you'll probably never notice. For a home or small business, it's like bringing a bazooka to a knife fight -- It's awfully costly to have that level of overkill, and you have the exciting possibility it could all backfire horribly...
And, of course, repeat after me... "RAID IS NOT A BACKUP STRATEGY." RAID will save your data should a single disk fail. It'll do nothing for your data should you get a virus, logical volume corruption, or encounter a user with privileged access who performs an act of drunken stupidity that includes an incorrect delete command... In fact, it's a LOT harder to recover from any of these scenarios if you have a RAID configuration. Plus, ALL of these are more likely than a disk failure, provided you run your disks in a reasonable environment. Repeat it again. "RAID IS NOT A BACKUP STRATEGY."
