Another "addon" I have recently adopted for the index card system is to use a clear desk mat.  When i get ready to work on a set of cards I can arrange them in a grid under the clear desk mat and not have to worry about them getting scattered or in the way if I want to use the desk.

EDIT: Just an update to say that i now consider a clear desk mat to be an essential component of the system -- it makes all the difference in the world when laying out a bunch of cards that would otherwise get in the way or be scattered throughout the day.