Generally the higher the number the better the card. The 670 beats the 660 and the 680 beats the 670. The core clock can be lower on a card and can still outperform another card with a higher clock rate because there are more cores on the one with the lower clock speed. The performance is also based on the clock speed of the memory and the memory bandwidth. There are actually many factors to consider, so the best thing to do is go by the numbering system the graphics card companies use, or go to videocardbenchmark.net to see performance results of a particular card you want to buy, an compare it with others.