is the point of the mod to make a sweet looking mini Xserve RAID or are you looking at this because you have a bunch of SD cards laying around? If its to make a tiny little RAID that looks just like a regular Xserve RAID then the SD cards will work fine, but I'm pretty sure you won't be pleased with its performance regardless of whether you split them up over several PCI cards. I am reminded of the Shuffle RAID that you mention, and also of the
http://ohlssonvox.8k.com/fdd_raid.htm floppy drive RAID.