They 6 and 7 are made from different level carbon fiber ("600" vs "700" series carbon). They have the same geometries and similar weights, but the 6 weighs just a hair more. By comparison, I have read that the difference between the 5 and the 7 is about 200-250g, depending on the size of the frame.
As for the forks, I think you'd have to check out part numbers. I'm going to go out on a limb and say that the 5, 6 & 7 series Madones use the same fork. Here's how I have come to that conclusion: The Trek website lists the 5, 6 & 7 series forks as being "full carbon", having E2 asymmetric steerer and carbon dropouts, but they don't list the type of carbon used, like they do for the frame. Again, just speculation on my part, but given Trek's past penchant for telling consumers whether a fork is made from RL, RXL or RXXXL carbon, to just saying "full carbon" leads me to believe that they can't differentiate between the forks in any appreciable way. If the 7 series fork were, in some way, better than the other forks, they would surely tout that.
Oh, and regarding the previous posts about tire size, the 2008-2010 Madones (second generation) could easily fit a 25c tire, both front and rear, and could have probably fit a 28, too.
Trek Crockett, Madone, Superfly, SpeedConcept & Cobia