1x 120gb (IDE - Linux drive)
1x 80gb (IDE - Windows drive - soon to be USB, since I'm getting rid of Windows)
4x 160gb in RAID 5 = 480gb (IEEE1394 - media storage for HTPC-kinda stuff)
1x 160gb (wife's computer)
1x 60gb (laptop hard drive)
Interpret that as you will. My next RAID will probably be 4x 250gb USB drives. Finding the right enclosure is hard/expensive, though. Performance is not terribly relevant for media files, IMHO - even a "big" bitrate like 24mbit/s is a whole 3 mbyte/s. That's hardly a problem on USB2 or IEEE1394, even in software RAID.
I generally agree with the comments that buying a single drive for your data is asking to lose it. Sometimes this is OK (your OS, apps, and games can always be reloaded), but for tons of media content, even if you do own the original media like we do, RAID 1 or 5 is the only way to go. It is my own personal experience that people are just not fastidious enough about backing stuff up to do without it. Anecdotally, I've killed 3-4 drives in the past five years. I don't really trust hard drives at all any more.
Finally, don't get hung up in the whole "terabyte" craze. It means absolutely nothing. Buy what you need.
-Erwos