You should read, at the very least, the first post in this sticky:
http://www.bikeforums.net/showthread...estion-MTB-FAQ
It describes the different uses for different types of bikes.
I could guess that you are more of a Trail bike candidate than XC because of your BMX background.
I could also guess that you would end up going XC, because most converted roadies do this.
Why should I guess, though?
You should read the sticky and learn about the different bike types. It's a bit more involved than hardtail vs. full susp.
People might be able to give better suggestions if they know more than just your price range.