They are lying, your performance will be severely reduced. Don't do it!!!
Purchase an ATI card like all good people do.
You know if you want to be helpful, don't miss lead people. Not everyone knows that your giving your opinion only rather then stating a solid provable fact. I do own ATI cards and find them incredible, but Nvidia also make incredible cards.
Your configuration will work great as it stands,and although the 480 is a great card , you will be limited if you ever choose to upgrade in the future to a two card set-up. PC's are very personal to each person that builds them, but I feel that a comparable AMD card may benefit you in the future depending on your needs of course, and what type of games and what detail level you choose to use with your software of choice. As we all know 2-3 years is a basic life cycle of a gaming PC, so just think about it for longevity.