the dimensions of this badboy are...
In case you don't know, these drives are DAMN expensive b/c they can only have 1 platter, so the density (proportional to cost) has to be super high, esp. on the large drives (40gb +). I'm not sure what the exact going rates are, but I'm sure that two 80gb 2.5" drives are much more expensive than just 1 160gb 3.5" regular one... ebay would be your cheapest bet.
I just measured the original 8gb the xbox came with, and well, it's so close to 1", I'll call it 1". If you wanted to make a fan sandwich with two 2.5" drives, you'd have 1/4" left for a fan, but there would be no room for any gaps on either side of it (so pointless). You could get a couple small fans (maybe laptop surplus/refurb parts) that are 1" or less in diameter and have them on the sides blowing towards the drives or something. I'm sure an indepth parts search would yield some pretty interesting devices. You'd probably also want a couple 2.5"-3.5" brackets so you can bolt the drive into the xbox tray. With those brackets, it would be very easy to stack them, and possibly bolt'em together nice & tight.