Good low-end mountain bike?

It seems like you've picked a price point, and from your question I don't think you're asking for something unreasonable.

PART I
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With that decision done, I'd say your next step is to visit a few local bike shops.

It's tough to recommend a specific bike (or even brand for that matter).

The most important thing is to go TEST RIDE a few. Then a few more. +/- All the bikes in this or (arguably) any specific range are going to be of equal quality.

What you want to do, is find one that *feels* right to you.

You won't know what I am talking about at first, but after a few test rides, you'll find one that just "feels" better.

Congrats, you've just found your bike. Now go test ride the other contenders again to make sure.

PART II
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Also, probably more importantly, just like you'll know when you find a good bike, you'll know when you find a good bike shop.

-- Their first questions will be to determine what kind of riding your really going to do.
-- They won't tell you to get full suspension. (this is specific to you, not a general rule)
-- They'll let you test and test and come back tomorrow to test ride again.
-- Before each test ride, they'll "fit" the bike to you.
-- After they see you ride off, they might offer other suggestion (you can tell a lot by just watching somebody pedal out of the parking lot!)
-- It will be hard for you to explain why you liked X better than Y, but they'll do their best to understand and tell you what you are noticing.
-- They won't be selling TVs in another department

BTW, considering it's summer you might be out of luck, but shops carry previous year models at some pretty substantial discounts. And there is probably a bit of wiggle room on the price of both "old" and "new."

Maybe a better question here is what's a good local bike shop? (I'm 500 miles south of you, so I cannot quite help there).

Once you find a good bike shop, your first question will take care of itself.

To Mike's point, any locals have suggestions on other good bike stores not too far from Cupertino?

I went to Bicycle Outfitters in Los Altos but wasn't too wowed...

I haven't been riding the Trek 4300 too much these days. But the one gripe I have is that not all front/back gear combinations are allowable. In some combinations, the chain rubs and sometimes slips off. I was warned by the sales person that I would have to live with that, no amount of adjustment would give every combination. In the high end bikes I would expect better gears that allow every combination. Other than that, the brakes are pretty good (at least on level ground) and the front suspension has the right amount of sponginess (it may or may not be adjustable).

Now then, why go to all this trouble? Because if you go to Craig's List, or Ebay, you'll find guys who got interested in Mt. Biking for about a year or so and rode their bikes 3 times. They spent $1500 on a lower end true X country Mt. Bike ride, with a good fork, great components, etc., and now they're selling it for $500. So, you'll get a great bike that you can actually take out on the trail if you ever get into it, but you could also just use it to dork around on the streets. It will take more work to go this route, but you'll get a good ride. Also, if you can find a gear head friend of yours who is into mountain biking, he/she can help accelerate the process.

Good luck!

I spent some time on MTbreview.com and must say all the terminology is pretty daunting. I may ping you as I get closer to picking something...

PS sorry about the missing 'edit' -- we'll add soon.