I'm just about in the market for a new bike, since my 4 year old pos walmart bike is on its last legs, and I don't really know how much I should spend.

If I get it with most of my next paycheck, I can spend about $300-350; if I wait until the next, maybe $400-450.

I like the specialized RockHopper, but would I really need that nice of a bike?
Most of my riding is done on fairly good surfaces, nothing you couldn't drive a truck on.

I'm almost 15 now, and 5'8 (weigh 130lb), and I'll probably grow a few inches, if I get a bike with room to grow, would it be bad for the time being?

On another note, how much would I be looking at for a chain/lock system to lock the bike to a bike rack (it wouldn't be in a 'bad' neighborhood, so it doesn't have to be impossible to steal, but I don't just want to leave a large investment out for the taking.