My one bit of advice from someone who is new to the mountain biking scene... If you know you want to do some legit trail riding w/ rocks, branches, etc., definitely just save up for a good full suspension.
I saved up some cash and dropped it on a Trek Cobia 29er hardtail, and while the bike rode great, as soon as I started getting in to some legit trails I was wishing for some forgiveness in the back end.
Ever since it was stolen I've had my eye on a Trek Rumblefish... I just can't find a gently used one near me!