Oh and bike brands are a little deceiving. The "brand" often only supplies the frame, with the shifters, derailluers, wheels, etc. made by other companies and often common to all bikes in a certain price range, with minor differences. Especially on the low end, there are very minor differences even between frames.
What you want to look for is a frame that fits. Beyond that the parts should function and stay in tune. Low spoke count wheels are a minus in your price range even though they look cool. I'd you're serious you'll want to at least get toe clips if not clipless pedals.
Realize that if you really enjoy it the first bike will be a learning experience. It'll teach you what you like and don't like so your second bike really makes you happy. So don't worry too much about getting the first bike perfect.