What is the difference between these two cards?

It is not possible to tell, from the info provided by amazon, if the two cards are exactly the same. They are the same make and similar models, at least, and both appear to have a reference design cooler. But there still could be differences. Perhaps one is a later version, which usually does not mean too much, and perhaps one or both are factory overclocked (and could be OC by different amounts), but usually if a card is OC it is mentioned in the listing since this increases its value.

For more information about the best cards, I recommend this THG article stating their recommended buys for different budget levels:

http://www.tomshardware.com/reviews/best-graphics-card,...

Beyond that, you might want to go to upgradevideocards.com and read about upgrading and the additional links they have for comparisons and reviews. These two pages might both be helpful:

http://www.upgradevideocards.com/gaming.html
http://www.upgradevideocards.com/reviews.html