From what i have read, people suggest going to LBS and riding a bunch of different bikes to find out what fits/feels best to you for the type of riding you will be doing. Obviously you would not want to buy a down hill bike for cross county riding.
There are a lot of good brands out there. Trek is a solid brand along with gary fisher, specialized, jamis, scott, giant, cannondale, Santa Cruz, kona just to name a few.
Personally, i am a bargain hunter so i bought a good used bike so that i could buy much more bike for my money i looking to spend. This may be an option for you as well.
mtbr.com has a review section as well as this forum. This is a good way to do a little research on bikes that you are interested in.