Avoid big stores like the plague. I went on a ride last Thursday with some friends, and two newbies on Wal-Mart bikes. One wasn't too bad except for a very loud and persistent squeak, but the other was just horrible. It didn't even nearly fit him - their solution was to sell him a taller seatpost, which on a MTB had him bent over like he was in drop bars. You could tell the suspension was utter crap as all of his pedal strokes were wasted on the rear suss flexing. My legs actually started hurting from having to coast so much, and I don't think he was feeling very good at all either.

My suggestion is either the Trek 820 or a used bike. Check with your bike shops, most usually have something used in stock. If not, the 820 has some very nice features for the money. With the '07s on their way in, they may even be marked down.

I personally highly endorse the Specialized Hardrock. I just picked up a '07 three weeks ago, and damn near 200 miles later I'm still in love. The front fork is going bye-bye, but the rest of the components are just fine until I break 'em .

Really, avoid box stores like the plague. I picked mine up at a semi box-store (Scheels sports) but ONLY because I found probably the gem in the rocks as far as salespeople. He was able to properly fit me to everything, and actually recommended the '07 Hardrock over the '06 Rockhopper (same crank, very similar components, and for a bigger guy the Hardrock is just tougher). Their service folks are top notch, I could tell that when I saw one of them leaving to ride home on a very well-used Surly Long Haul Trucker, complete with duct-taped saddle and brooks grip tape.

Building a relationship with a shop is also good. Scheels may be "big", but they are somewhat local. When I had to avoid an idiot in a Toyota and hit a curb awful hard, I took the 'rock in expecting a large bill for re-trueing the front rim. Nope, they took it back, came out 20 minutes later and said "have fun".

That's my advice. Also, stick with your budget. Don't over-stress it if you can't afford something nicer. The Trek 820 is a great starting point, IMHO better than the 3700. Steel rides much nicer than aluminum, and the 3700 really doesn't have that much of a component edge over the 820.