i wouldn't bother spending less than six or seven hundred on a bike, especially if you're buying it new. in action sports more than anything, you get what you pay for and you want to be confident in your gear. i've seen cheap bikes legitimately fall apart in the middle of an intense dh ride, and it's not pretty. a good starter hardtail would be a Trek 4 series, i still use mine for cross country/trail riding. it's a good, reliable bike that isn't very expensive. however, your best bet is to go to the nearest bike shop and ask them, they'll tell you exactly what you want and they'll set you up. some shops will even do small repairs/part replacements for free if you bought your bike from them.
DON'T GO TO SPORTS AUTHORITY, DICKS, OR ANY OF THOSE BIG CHAIN STORES. most of the time they don't know what they're talking about nearly as much as real shops do, and a lot of the time the product isn't really what it says it is.