I just found a link to Ben Pridmore's description of his (and many others) memory technique:
<a href="http://memoryconsulting.com/pridmore.htm">http://memoryconsulting.com/pridmore.htm</a>
Ben is World Champion in many of these memory events.
He describes how when viewing pairs of cards, he uses "2704 different images. For numbers, I use the same list of images, but only use 1000 of them for decimal and 1024 for binary"
So with digits he gets speed by "chunking" several together (~1000).
Richard