For a system drive, RAID 1 (mirrored, for safety) makes more sense than RAID 0 (striped for speed - but more than doubles the chances of a failure.)
I recently built a fast video PC at work, and used a Raptor for the system drive. Frankly, if it boots faster or runs faster, it's hard to tell.
For video, I bought a 4-drive RAID. I run the first half as RAID 0, for speed. I set up the second half as RAID 10 (two pairs of striped drives for speed, mirrored for reliability). I'm using this second half for backing up my source and end video files.
I'm happy with the RAID setup, but looking back, I should have bought two cheaper drives for the system and mirrored them for safety's sake.
These days you might install an OS drive and partition it for
1) Windows XP - it's the most efficient MS OS for media
2) Windows XP64 - Lighter than Vista, but ready for 64-bits. But will it get long term support?
3) Windows Vista 64 - Someday...
In the meantime you can use 2 & 3 for storage, but you'll be ready to move the storage content to another (larger, cheaper) drive in the future, and then you can install your future OS.