The faulty items are definitely covered under the Sale of Goods Act.
The 7 days they quote from the Distance Selling Act is a cooling off period, and would be relevant if you do just want a different card. See here for more info -
http://www.oft.gov.uk/Business/Legal/DSR/default.htm
This doesn't affect your rights under the Sale of Goods Act if the goods are faulty.
I'm not sure how this relates to the product description being incorrect. You could probably argue that you didn't use the DVI connection straight away and therefore didn't notice there was only one, but now you want to use them and have discovered the inconsistency.
You may want to check out the Trade Descriptions Act as I'm not sure how about time limits for claiming. If they are still listing the item incorrectly it is covered under the act as an offer to supply goods.
Trading standards will probably be able to give you advice on how to proceed with the TDA.