OP find a local bike shop you like, Trek,Giant,Specialized, matters not, But you will NEED the support of a LBS.
Buy the Hard Tail bike they tell you to buy and ride the thing until you are under 225 pounds,, beat it to death,,then get a better bike
It will be worth the $500-$600 you will spend, so keep saving your pennies.
Most/ALL $500 hard tails come with a crappy suntour fork made for mom and the kids to ride a dirt path..
There is an Upgrade program,, Do this as soon as you break the fork,, relax it will only blow a seal and leak,,,
http://www.srsuntourna.com/upgrade/
Spending $300 or so ONLINE or at Dicks sporting goods will get you a shinny bike,,,that you will hate after a month..
Sorry but a decent bike you will benefit from is going to set you back about $750 ish,, bite the bullet and just do it !
I'm sure you've blown money you did not have on wild women, flashy cars and parites before so man up and face the music !
Mountain biking Is worth it,, are you ?
You will live longer and stronger,,put a price on that !
TANSTAAFL
There Ain't No Such Thing As A Free Lunch..
Do not re learn the oldest of lessons over and over again,,,, You get what you pay for In life ~
Oh and make sure you wrap the chain stay with a Lizard skin or an old tire tube,,trust me on this, A good bike shop will back me up here. I hate rattles and chewed up chain stay's, your next bike will have a clutched rear derailleur I hope..