All RAID (except for spanning RAID), goes by the lowest size HDD you have.
RAID 0 - Theorectically doubles the speed of your hard drives. You would lose most of the space on your 40 GB, and in essence have 2x13=26GB of total space. Low reliability.
RAID 1 - Mirrors the data, so that if one hard drive fails, you're still safe. Again, you would lose most of the storage space on your 40GB, and end up with 13GB total. Mirroring does not take advantage of all your storage space, since storage is redundant.
RAID 0+1 - Does both of the above. 4 drives are needed. If you added two more 40GB, you would still only have 26GB of storage space.
Spanning - Just creates one hard drive out of multiple drives. Basically the opposite of partitioning. You'd end up with 53GB total.
These are all the options for IDE RAID. If you went to SCSI RAID, you would have more options (like RAID 5).
I'd provide you with my normal source for information on RAID, but their site appears down, so I can't get you the link.
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