I was in the same exact situation as you a couple weeks ago with the same budget. I did ALOT of research for about a week and a half. I finally decided on the '08 Trek 4300 Hardtail, but when I got to the store I ended up taking the next model up(4500).
I haven't actually taken it on a trail yet, infact I have only ridden it home the day I baught it due to the crap weather we've had here in NJ. I'm really happy with my purchase though and would definately recommend you take a look at both of those models. They both come with disc brake availability. The main advantages to having the 4500 is that it comes with front fork lockout. So if your riding on the street, you can lock it for less effort on when pedaling. Also the 4500 comes with better trail wheels. Go onto the Trek site and they will let you compare one with the other.
My 4500...




TREK 4300:


TREK 4500: