Actually the 2 x 250 are already RAID 1.
I have had an xtra 250 which I have used as external. Now I've bought a 500GB and thought of either use it as RAID 1 with the 250 that I had as external, e.g put them together internally and loose 250 GB.
Later on in the future, when needed, get an extra 500GB and couple it with the 500, using full capacity. Then taking the 250 out and putting in the external box.