You could use RAID software built into either the OS or written by a 3rd party, but then the array would be non-bootable (chicken & the egg). You could also by an adapter for one of the drives and hook both to a RAID controller.
think thats a good idea, from what I understand, I think the SATA implementation at the OS level would prevent a relaible software RAID accross the two (though I could be wrong)