Any of the "big" brands should do you just fine in that range (ie- brands you'll find in a bike shop and not a department store....Giant, Specialized, Trek, DiamondBack, Fisher, etc). Do NOT buy a department store bike. There's a reason they're so cheap.....it's because they are complete crap!
At that range, I would stay away from any full suspension bikes you may come across. It costs more money to make a f/s frame so they will use cheaper components elsewhere to make up for this. An aluminum frame is also more expensive to produce than a steel frame, so same logic applies there. Alu is getting to be pretty common as bike frame material tho, so the "hit" to the components on the rest of the bike will be much less than for the f/s bike. I personally like having an aluminum MTB as you can just hose it down after a muddy ride and have no worries about the frame rusting.
Hit your local library and see if they have back issues of Bicyling Magazine or maybe Mountain Bike Action and dig through them for reviews of lower-end bikes. They'll usually do a good job of breaking down why a bike is a good value for the money or not. I think Bicycling has a sister mag called "New Rider" (or something similar) that is geared to new cyclists...might be a good source for entry-level MTB reviews.
At the price range you are looking in, they main thing to be concerned besides proper fit (very important! If the bike store sales guy seems clueless on this, find another store!) is what level of components you are getting for your money. Take a minute to browse through
Shimano's MTB components page and familiarize yourself with the heirarchy of the component "grouppos". That way you will know what stuff is "better" when shopping for the bike. Let me know what you find and I'll be happy to offer my input FWIW.
Good luck, let me know if you have more questions.
Fausto