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.