Well, if you are asking "what is the most performance I can get for $500", then I would say the answer has to be the 6950's in crossfire. That is according to this article.http://www.tomshardware.com/reviews/best-graphics-card-radeon-hd-6990-geforce-gtx-590,2912-6.html
A single card solution of the same performance will cost around $700, so no a $500 single card solution will not be as powerful as the crossfired cards.
However, I agree with what jockey said. Unless you demand that much video muscle right now, I would buy a single 6970. Then wait 6-8 months to find a big sale on them and pick another one up. Maybe by then there might be more games that demand the power of 2 6970's.