For your price I would suggest
http://www.bikesdirect.com/products/gt/gt_nomad_2.htm

Or
http://www.bikesdirect.com/products/dawes/eclipse2.htm

The Nomad has bigger tires, so it would be much better for dirt roads. The Dawes would be good too, but I wouldn't want to ride it on rough roads. Smooth dirt roads should be okay.

Unless you find a good deal on craiglist for a used bike.

For your height, get the smallest size. XS, S or 15 inch frame or 14 inch frame.