Chairman Ray said:
The duo 7950s will outperform the single 7970, but I would recommend the 7970. You don't need anything more powerful than it right now, and a few years later you can buy a second one for cheap.
In a few years when he starts looking to add a second 7970, he'll probably find that a mid range card from the current generation at that time will be more powerful and be a better value than the second 7970. Waiting a couple years to add the second card never works out.