Sonnettech market a range of SATA 6Gb cards for Mac, which will support SSDs either internal to your Mac, or in an external enclosure. The 'internal' cards allow you to mount the SSD directly on the card itself, as the Mac Pro's (otherwise convenient) drive caddies make it difficult to connect internal drives to the ports on a PCI-E card.
RocketRaid also do a SATA III card for Mac, but I wouldn't recommend it. I don't have experience with the Sonnettech product, but they seem much more committed to the Mac market.
If you're adding an external enclosure, please take care not to buy a model with a SATA II interface (sometimes called a 'port multiplier'). I found a model from Addonics which has no real SATA hardware on board - it just does passthrough directly from your SSD to your Mac, so the SATA III throughput is not hampered. I don't believe any of the Sonnettech external enclosures are suitable for this.
The Sonnettech card with four externally hosted SATA III drives should be able to push you over 1GB/s of real throughput.
Good luck with FCP X. It's a lot of fun to edit with.