I hate to answer these kind of ??? 'cause it's easy to offend or come across as a jerk, but i'll try 'cause i've been in your shoes.
my advise would be to save more money and get the best bike you can afford. i have an entry level bike i bought for about $650 and after dumping a shit ton of money into it, i still had an entry level bike. Why? the frame. no matter how many upgrades you put on a cheap frame, you still have a cheap frame. a Trek 3900 frame wasn't made for jumping, so no matter what you do to it, it will still be an entry level bike.

Good Luck.