Be sure to check the correct Cannondale geometry chart for the specific cross bike you're looking at too....The geometry page above is for Cyclocross 2, 5, and 6...The Cyclocross 4 (Tim Johnson edition) and 7 are a little different.
As for sizing, I ride a 52 Cannondale Six13 and System Six. For a cross bike, I went a size down (as recommended by Cannondale) with a 50 (Cyclocross 4) and it fits like a glove! Despite being "smaller" (both in labeled size and top tube) the bike does feel "bigger" and "beefier" than the Six13 and System Six...if that make's any sense...
The cross bike's geometry is pretty much completely different than road (longer wheelbase, slacker head tube angle). Cannondale clearly has their stuff worked out.
I'm proud to be a Cannondale shill....