To the OP and other "newb",
I agree that a good full suspension design is your best bet for maximum fun, but you can get a lot better components and a lighter frame for the same money by looking at 26" or 29" hartails. For your first mtn bike purchase, you really should go to a shop and demo bikes in your price range. Just like skis and boots, each manufacturer uses different geometry and materials for their frames and you need to find out what is most comfortable and efficient for your body type. Each bike will also have a different feel in terms of ride quality and handling characteristics. Highly recommend buying from a store the first time and developing a relationship with said store. For a truly trail-worthy bike, look for components no lower than Shimano LX (LX, XT, XTR, Saint), or SRAM X5 (X5, X7, X9, X0), anything else will not hold up well to the abuse it'll get off-road.
Your store should offer a free tune-up for your bike in the first 30-90 days of riding, as well. Eventually, you will need service and advice so buying at a local shop is a good way to learn all you need to know about your bike. When you purchase your bike, get some chain wax and lube, make sure you get a run-down on how to clean and lube your drivetrain and cables, and make sure you do this religiously, after every ride! Doing this will keep your ride running like new for a very long time (barring the inevitable crash damage
I know it's tempting to go used or e-bay, but without understanding bike maintenance, the parts that are likely to be shot, and weaknesses in frame design, often a used bike costs enough to repair/refurbish that it ends up being cheaper to buy new (with a warranty). Once you've learned to tune-up and repair this bike and it's time to upgrade, then e-bay it by all means!