I worked in a bike shop for 12 years, I can steer you the right direction if you have questions. Fitting a bike is like fitting a shoe, make sure you get the right one or you will never be happy with it. from your description a 21" frame should be a good start, different companies measure bikes different, so it would be worth your time to test ride a few different bikes.
As far as what you get for your money. Bikes in the 250-500 range are good comfort / entry level / easy trail ride / cruise the neighborhood bikes. 500 - 1000 range get you some more performace / recreational bikes. Something that you can ride some of the tougher trails with, hit moab hard with, even get you started in racing if you wanted to. I would just ride a few different models and makes in your price rance and see what seems to fit the best and have any features you like more than others.
As far as brands go. There are lots of companies that all compete for value. So you will see many similar features maybe some slight variations, but most of them will be faily comparible. I would look at the big name brands in that price they seem to have the best for the money. We sold lots of Specialized, I would look at those first. Also check out Trek for sure, they are always competitive on value. Gary Fisher (made by Trek) is another one. Schwinn has a pretty strong name again. That will get you started at least.
Also look for some of the last year models. New models prices the dealers hold their price on pretty close, last years often will go for much less and often are basically the same bike. If you have any questions post them up, or PM for a phone number I can always talk bikes.
yeah, I was single once. 
