Well it's hard to give you any specific advice sine you aren't really sure what your price range is, but since you've taken such a quick knack to mountain biking & have a steady, decent paying job (assuming you don't have a wife or kids to support on military pay), I would look into something closer to the $1000 range, maybe even $1500 so you can get a good full suspension XC bike. There are plenty of bikes much cheaper that would be fine to begin with since you didn't have any troubles on the Trek 3700 (about about $400 new), but if you can afford it & know that mountain biking is a sport you're really getting into, it's cheaper to throw out the extra money now instead of having to upgrade later.

Now, that said, go to all your local bike shops (LBS). Think of your new bike like a new car--all the salesmen want you on their bike, & probably honestly think they're the best option for you. But only you know which bike feels right. What they can all do (since most bike shop guys are much less sharky than car dealers) is help you assess what kind of bike you really want. I went to mine I said I do mostly light to intermediate XC, & he immediately recommended the Hardrock Sport Disc as the best value in that area, even though it was one of the least expensive bikes in his shop. But just shop around & ask lots of questions, because mountain bikers in general are pretty passionate about the sport & your LBS will be more than happy to answer any & all of your questions, & get you on your first real mountain bike.