Fork question for Specialized expedition sport

Alright, I ride a specialized expedition sport. I've owned this bike
for almost 2 years now. I really like it because it never causes me
any trouble, and is light enough (29 pounds) to be just fine for
recreational use. Anyway, I won't have nearly enough money to
purchase a new bike for maybe 3-4 years, but since I ride every
day(that is possible at least without missouri mud), I would like to
replace one more thing on my bike. It already has had some standard
things replaced, like the steel handlebar, and the seat post which
weighed about as much as a gorilla. So, due to the time I spend on my
bike, I would also like a better and lighter fork. Anyway, apparently
(so I'm told) my front fork has about a 68 mm travel or so. The guy
at the local bike shop suggested that if I want to replace the fork,
that I find a good deal on an 68-80mm at the uppermost range travel
fork. He said anything greater than that would mess up the geometry.
It seemed to make sense the way he explained it. He said he thought
that Manitou made a 80 mm Black, and that Rockshock had a Psylo in the
80mm range as well. He also mentioned Marzocchi. Anyway, my question
is, when I look around at the specs for these forks, they are usually
80-100mm or 100-120mm. I'm not having any luck finding anything even
on the manufacturer's websites. So, is it even possible to find
anything--am I looking in the wrong places? Also, how badly would the
geometry be "messed up" with an 80-100mm fork, and in what way? I
really don't know much about the geometry of my bike let alone bikes
in general, so I can only learn, but since the LBS said that a
80-100mm fork would bring the front of my bike up too high probably I
wanted to see what you guys/gals say.

Anyway, any help is appreciated. And thanks to all who helped me with
my Hayes brake issue--it is now perfect, and I love it.

Thanks,
Alan