I have to agree with zenmaster here. I've been researching setting up a a 4 or 5 disk RAID 5 setup for my media files, since I want a TB or two of storage and I'd like to not have to backup the data on DVD since that would be reams of disks. Once you're up at 4 or 5 drives, the percentage capacity hit you take from RAID 5 is less, since it's still only one drive.
However, I had to consider one simple fact. In my time, I've never lost a HDD. I've known people who have, but it hasn't been a terribly common experience. On the other hand, I've burned at least two motherboards. It seems to me that as long as Mobo's are less reliable than HDD's, then there's not much point to worrying about setting up a RAID 5, or any security RAID for that matter on a home system, unless you also buy a second identical motherboard or RAID card and keep it in storage.
No... when I do build my media server, I'll be using a file system that allows dynamic adding and subtracting of drives to a given volume. Modern file systems can bring you some RAID-like performance/security features without the hassle of using RAID. Downside is I don't think you can use Windows with such file systems. Microsoft is slowly learning to play better with friends, but it's painfully slow. Linux makes more sense for a file server anyway.
For the OP: I believe the consensus is that if you want performance HDD, then the best bet is to get a good 10K (or even 15K are available now) RPM drive, along with a standard 7.2K drive for file storage.