Faster hard drives (10Krpm) don't have the storage capacity of 7200 rpm drives, which make them not-so-ideal for handling much video...
I have a software raid right now, consisting of 2 300 GB Maxtor drives (installed in bay 2 and 3 of my Mac Pro). I have this raided drive set up as my scratch disk for Final Cut, and keep all my media on it. Final Cut is definitely more responsive when working with this raid, as opposed to working with media on a non-raided drive. I say go for it, just make sure you back up on a regular basis.