cyclezen, Having done EXACTLY what you propose to do, I think you should go with the fork you want. I upgraded my son's 2001 Hard Rock from a 75mm fork to a 100mm fork. I checked with the folks on the mechanics forum. The only concern with the change was the potential stress on the head, and no one was really concerned about that given the extent of the change in fork travel.
Consider some practical application points. You're not riding an air fork so consider that on a spring fork, there is a difference in riding geometry between a 150lb rider and a 200lb rider because the same fork compresses more when the rider mounts.
After making the change from a SR Suntour spring fork to a Manitou Axle with oil rebound and HD springs, I rode the Hard Rock on the local technical trail. The bike handled great. The superior qualities of the better fork (and I know the Axle is an entry level fork) more than made up for any geometry changes (yes the Manitou lifted the front end about 3/4"). I would be more concerned about obtaining a quality fork than increasing by 20mm. But I would not push it to a 120mm fork. At some point the stress factor on the head does become a concern.
As for buying a new fork, I didn't twice. Instead I purchased used F-100 Fox air forks for both of my MTBs. I paid $220 with shipping for a standard F-100 and $250 with shipping for an F-100 with terralogic, both off ebay. Both forks have performed well, but the standard F-100 was exceptional. BTW, now that I've gone air fork, I won't go back.
You need to go the way that meets your needs, but don't allow a small change in dimensions to force you to buy a fork you're not happy with.
I'm attaching a pic of the Hard Rock after the Manitou was added.
