I think it depends on your intended use for the PC. Gaming will generally dictate a more powerful card. There is a fantastic website called gpureview.com which lists lots of specifications for all types of video cards. It is very well designed, and has side by side drop down lists to let one compare two cards to each other.
It also has detailed info on the GPU chips in those video cards, as well as power usage generally required for the cards. Spend some time there, and you will almost be a PRO at video cards!!!