mountain biking is my new favorite hobby. Definitely check out
http://www.morcmtb.org/ if you want some people to hit the trails with or more beginner mountain bike tips.
Ive got a Trek 3700 with front suspension I could sell you for $100, it was my first trails bike but I ended up getting a full suspension bike. (which in retrospect im not sure was a good idea)
You don't really need a full suspension bike to ride the trails, a lot of people ride with NO suspension at all.