Finally I can add some assistance in an area I have / had extensive knowledge. Taking into account I have not been into mountain biking as much as I was before my children were born three years ago. This 3 year old info is what I have to offer.
Bikes are built like computers. You take some deraileurs from one company, brakes from another, seat from another, tires from another, and so on. The only part of the bike built by the company you are buying it from is the frame that holds all the pieces together. On most bikes you will be looking at, you will see Shimano Deraileurs. Shimano makes all different levels of deraileurs from ones you might see on a K-Mart bike to a $3000 titanium custom.
When I was involved in the sport the Deore XT was the top of the line. Deore was the next level down and there are others falling in place below that. The main advantage with getting higher price level deraileur is shifting accuracy (the difference between the feel of a Chevy Citation gear box verses a Ferrari gear box). The rest of the components from other manufacturers should be judged on how they feel to you (test ride many bikes)and their perceived quality (fit, finish, etc.).
The other factor is overall weight (over thirty pounds is heavy these days, low twenties is light), and especially rotating weight (Tires with steel or Kevlar beads. Tall knobs, heavy or short knobs, light).
The main question is: How much and how hard do you intend to ride.
I used to ride 80 - 100 miles a week. I could justify spending $1K on a Trek 8000 that is race ready for any local race. My wife rides 80-100 miles a year we spent about $400 on a Trek 420 and it makes perfect sense for her.
One last note on suspension. Front suspension is a gift from the gods. Don't be afraid of stepping up a level or two from the bottom of the line if you are going off road in an aggressive manner. I'm split on rear suspension. It feels great, but you sacrifice a lot in weight and component level to get it. You have to spend significantly more to get the same level of components and weight as you would have without the rear suspension.
Unfortunately, there are a lot more fine details, but you will pick more current info going to many bike shops and taking a bunch of test rides and talking to the sales people.
Best wishes,
Kent
P.S. I my experience expensive deraileurs bend just as easily when you hit a stump as cheap ones. [:)]