For $100 more, you can get a good entry-level bike from a reputable dealer that will stand behind the products they sell. I started out with a Specialized Rockhopper with 29″ wheels for $549 (USD). Not only is it well-built and upgradeable, but the dealership provides me with free annual tune-ups and free labor for the installation of upgraded equipment and accessories. They also made sure that everything on the bike was set to my height, ensuring that I had everything where I needed it to be before hitting the singletrack.
I cannot tell you how invaluable it is to buy from certified professionals who can ask you not only what you want out of a bike, but can also educate you on things you may not even be aware enough to ask as you get into this great sport. Like you, I just wanted to get a bike and get out there, and I was looking at department stores and online. I stopped by my Specialized dealer out of curiousity, thinking that there bikes were out of my price range. I learned that I was wrong about that, and now I have a relationship with experienced riders who know my bike and can answer any questions I may have. It truly has made my entry into mountain biking far more enjoyable.