you can, but they say that the additional seek times, differences add up.   I have two 30 gig 75GXPs as raid 1 now, they were raid 0 and they were built about a year apart (different "versions" I'm sure) but ran ok in raid.  I doubt that the differences cause any increase in CPU useage, probably just a little extra delay in data because of the number of times one drive will be waiting on the other....