If you're on a MacPro or a dual G5 with all the bells and whistles (RAM GFX +++) you can get more RT out of the box in case you're editing HD or uncompressed SD.
If you're editing DV, you won't notice much.
A RAID will anyway speed things up more than single disks, but it can be risky if you forget to back up.
I'm just importing SD QT/mov files that I've recorded to an nNovia QuickCapture drive or a PC laptop running DV Rack. Editing on a 1.8 DP G5...no bells, no whistles.
So I guess the only reason to consider a SATA drive or RAID would be to future-proof the investment, in case I ever graduate to a more demanding format.
Best solution is a secondary SATA inside the machine for what you want. Get a 500Gig or bigger SATA to put inside the machine as your secondary/media drive. FW drives will work just fine. You won't notice any difference with SD, RAID will give you no advatages.
Drive speed in FCP tends to be a binary switch - it is either fast enough or not for the task at hand. Better drive performance will get you more RT tracks, but seek time is more important than sustained transfer rate there. SATA is faster than FW800, esp. for RAIDs.