Any major brand will do. Go to a real bike shop, not a department store. If you get a cheap bike you will hate mountain biking. Go to the upper end of your range. Trek, Specialized and Giant have bikes in the $600 range. I had a $300 Sears bike and quickly realized I had wasted $300 once I got into riding trails. I decided to go full suspension and got a $1400 Jamis. There are some good full suspension bikes around $1000.
I have really enjoyed mountain biking. I got really into it around last April. I different experience than hiking. You will get some cuts and bruises as you learn though
