Similar to my system except I have kept my OS slim enough to do compressed disk images instead of clones. Course I don't have Garage band or iDVD installed but I am keeping my system down to 5.5GB.You would be better to just partition the external into 3 and roll through them in order using erase in Superduper on the oldest.
SD remembers which drive you backed up to last so you just go to the next one. ) call them One Two and Three.
I do that with two partitions Odd/Even and then have a third backup once a month that is away from the computer on a dedicated drive.