I've been in the bike industry for about 25 years, and Kyle & Kari basically hit the nail on the head. There's a massive difference in quality between what the mass merchants(Wal-Mart, Target, etc) and an independently owned bike shop will offer. In a bike shop, you'll find the brands that you mentioned-Trek, Specialized, Giant, etc.(Gary Fisher was a sub brand of Trek, but doesn't exist any longer). These bikes are built to much higher standards of quality, and they're assembled by trained bicycle mechanics, not just a part-time hourly employee with no training. There's a huge difference in safety between the two. Don't get me wrong, department store bikes exist for a reason, but I would never consider riding one on real dirt, mountain bike'ish type of terrain-they will not hold up.
There's also no support on a bike sale at a department store, where a bike shop will generally offer some sort of maintenance package, warranty, etc. They also do not come in wide variety of sizes at a mass merchant, so you won't get a bike that actually fits you. People come in a vast array of heights and sizes, so bikes should as well. You'll get properly fit through a bike shop, and ride out on a bike for you.
My suggestion - save up as much as you can and buy the bike that's going to offer you a comfortable, safe, enjoyable ride. Something else to remember, every major bike manufacturer makes bikes in all price ranges, starting at about $350 for an adult mountain bike, up to however many thousands of dollars you want to spend-the sky really is the limit!:) My point - all the manufacturers use components from the same suppliers, for the most part, to hit each of these price points. There's some exceptions to this, such as Trek-they manufacture a lot of their own components(wheels, tires, handlebars, etc-Trek is the largest bike brand in the world) Go with a major brand from an independently owned shop, and you'll be happy!
