After having a talk with Oli Metcalfe this summer when he was with Muse (and his Catalysts) at Rock am Ring I tried the 3 x 500G SATA Raid Level 0 (Striped).
Works fine for me. Access time seems to be just a little slower than the 15K SCSI times.

I ran 2 full HD 1920 x 1080 Photo-JPEG 75% movies at 30 fps.

Starting another HD playback led to dropping frames on two layers.

But I could run another 720x576 Photo-JPEG 75% movie at 30 fps.

I will do some more testing with SD Movies on more layers.

System setup: Mac Pro 8xCore, 4 GB, 1x500GB System, 3x500GB Content