raptors are 150Gb these days.

doing this kindof thing - you need to be prepared to iterate a couple of times.
start small, investigate the technology. see if it works.

i dont know the answers about pci-x cards - i dont have any - if i were doing this - i would use an xserve raid - with fibre channel - because i know it works.
i have seen it working with my own eyes.
and its reliable, and its designed to do this kindof thing. and make sure that it all happens correctly- and it has a huge cache which makes the problems a little easier.