Hey all, I'm brand new to the forum, and figured I would ask some advice from those in the know before I go out and spend a lot of money...

I used to bike a lot as a kid and teen, but have been off of two wheels for well over ten years now. My girlfriend recently bought a Rans (man is that thing weird), and wants me to get something to accompany her on weekend rides. My idea was this (and you guys tell me if this is even feasible): to buy a mountain bike and then get a narrower rim and some road tires and swap the two, when necessary, for a "jack of all trades" (master of none) kind of bike. Can this be done? Is it advisable? A lot of my riding will be in town, but I feel that a mountain bike with road tires would do a lot better on the road than a road bike with knobby tires would on a trail. Am I right? Are there any kinds/brands of bikes that would be more suitable to this kind of setup. Any help would be appreciated? -J