Go read MTBR.com (mountain bike review). You should be able to get a good cross country (XC) bike with XT or similar level components, disc brakes for the price range you are looking at. I would suggest also going to WrenchScience.com and do an on-line fitting to find out what size bike would work best for you. Depending on how much climbing you want to do (are you doing this because you also want to push yourself hard and get into shape?) you might want to go with a light-weight hard-tail. If only moderate climbing, maybe a somewhat light bike with around 3-4" of rear travel with a shock that can be set pretty stiff and 4-5" of front travel.
A good shop will let you test ride bikes. We've even had manufacturers show up here at trail-heads for demo days (same as skiing) where we could test ride bikes on the trails we actually ride. Ask your bike shop if they know of any events like this.
There are good mass produced bikes out there from GT, Jamis, Gary Fisher, Trek and many others that should be in your sights.
If you aren't already good at wrenching on bikes or are handy enough to want to do it from the start, spend the extra few bucks and buy from a Local Bike Shop (LBS) that has a good reputation for working on bikes (ask around). You don't want to buy on-line or from a talentless shop and find that your bike needs work right out of the box, and not have someone you can trust to work on it. Usually a good shop will include the first year of service and tune/ups with the price of the bike.