Just to add a little more to this thread, I already own a hybrid that is suited to off road riding so I focused on hard pavement riding for my new Giant bike. Also, I HIGHLY recommend that you be realistic about adhering to a budget unless you truly don't have restrictions. I am not a rich guy by any means but I started out with a $1,000 budget yet I ended up spending $1,500. In the end I decided I wanted a composite frame and I also decided to pay extra to get it.
In regards to used bikes, I think it is a good idea. HOWEVER it depends upon your needs in regards to frame size. I need an XL frame and I found the selection of used bikes I was interested in along with the frame size I needed to be non-existent. I changed my focus to new bikes and I am glad I did. My LBS (Bike-King in Inver Grove Heights, MN) has been awesome.