Oh come on guys, you don't need to add more platters. Just cut the head in half lengthways carefully. I suggest a sharp knife but my neighbour used an ax, and he says he has even seen it done with an chainsaw! (No way I'm that brave.)
Now the head is half the size, you've doubled the density. So much cheaper as long as you has a steady hand.
Alternativly, get all those credit cards you nolonger use, pull off the magnetic strips and stick them onto the AOL cd's you receive by the truck load each month. You might need to either trim the CD, or make a new housing out of cardboard for the larger disk size. Beware. If you do this, do a low level format to ensure the credit card information is properly erased or else it can be accessed by all the virusus out there looking for credit card numbers on peoples machines. (If you don't have access to a low level format tool, soak the magnetic strips in a strong solution of bleach and amonia for 72 hours prior to attaching to the CD to ensure they are properly cleaned.)
Hope this helps,
Moohoo