To touch up on Zeph's last post: you may or may not have tons of fun doing downhill biking on any given style bicycle. The original mountain bikers were riding beach cruisers for Pete's sake. However, if you ever want to ride anything like you see the pro's doing, you'll need a bike similar to theirs'. The original mountain bikers were NOT riding anything like what is considered DH today.
Given the massive difference in the riding styles of DH vs flat land, your best bet would be to try to find a Kona Stinky, Specialized Big Hit, etc. around your price range & then save up $100-200 for a cheap rigid trail bike for the non-DH stuff. Either that or look for a decent all mountain bike like Zeph suggested. I would let the selection/deals you find determine which route you go.
And to clarify, I don't DH bike, know next to nothing about it, & say all of this based on what I do know about MTBing in general. So if any of the approved advisers flame or correct me, listen to them.