No way to know whats optimal until people can test throughput with a few cards of different types.
My base plans are...
1. to get the SDXC cards. San disk has 64GB 90Mb/sec cards which match my existing San disk CF cards. If there is a speed advantage I'll team the cards with data being written alternately (every other picture). Thus, no backup, but higher throughput and I can shoot twice as long. Since I shoot in bursts, if a card dies I loose every other picture, but still retain some of every group.
2. If there is no speed advantage of option #1, I'll still get the same SDXC card and shoot with the data being mirrored to both at the same time...when I get enough cards
3. If there is no speed advantage of option #1, I'll use existing cards (smaller slower) and save JPG to SD and RAW to CF. I might do this at other times, such as when I need quick access to the files during or immeadently following the event and I don't have time to convert from RAW to JPG.
That's just my plan based on untested theories, but they will get refined as things become more certain.