80mm to 100mm fork replacement, Yes or No?

[!TREK4ME!]

Hello,
I have a Trek 4900 with a Rock Shox Pilot C fork that is leaking at the seal. I'm not sure whether I can replace the seals and from a earlier post, it's probably more costs effective to just get a new one. I've got a couple forks in mind for replacement, but have a question:
The original fork is a 80mm... should I replace this fork with the exact size travel fork or can I get one with a little more travel(100mm) and hopefully not mess up the geometry of the bike/ride too much??
I'm a Clyde @ 6'-4" and 275lbs and the original shock was a little soft, but seemed to do alright. I had the preloads adjusted all the way down and had about a 15-20% sag when I put my weight on the bike.
I'm looking to upgrade to a lockout type, ex.--Rock Shox XC28 100mm or maybe a XC30, not sure yet.
What do you guys think??
Thanks,
PJ