I presumed youd have been running the 6950 as a 6970 as most did but i new bf3 would be brought up well thats one game and special drivers to make it run fast for that one game 95% of the rest it's not much better tbh thus i dont consider it a worthy upgrade it would be like going from a 7850 to a 7870 an upgrade but not a very big one.He would be better off sourcing a second hand 6970 for xfire which is faster than a 7970 you have the board and the power supply to do it so that's what I would do if you can source a 6970 for around £100 ish.
Actualy i haven't as mine would not unlock.
Its not just BF3 where the difference is significant,
http://www.anandtech.com/bench/Product/548?vs=510 <- and that before any of the 7### Drivers reached anywhere near what they are today, you can add anything from 10% - 30% to those comparisons, and then there is the overclocking diffrance...
He asked "would you upgrade from a 6970" my awnser having upgradeded from a 6950 is absolutley yes: reason; less power, less heat, much higher performance, higher overclocking and performance upgrades from the drivers is dead on the 6### series cards.
Yes he could add another 6970 for £100 but that would add another 200 Watts to his system and that much heat, and then you have the xFire game lottery, so why not sell the 6970 take that £100 he would have spent on the other 6970 add £30 to it and get him self a 7950?
No for those games where xFire works well in its not going to be as fast as x2 6970's but still 40 / 50% faster than what he has now.
Going xFire is one solution if he does not mind the extra power and heat in his case, but what he will end up with by the time he upgrades again is pretty old and unwanted tech.
PS: the 8### series cards could be a year away.