To be honest, you'd be fine with anything in there. You're best of sticking with single GPU setups in general though since there are still modern games that don't work well with dual GPU setups. I had a dual GPU Radeon so I know first hand! GTX660 would be an excellent choice, or GTX670 (depending on what you already have - you wouldn't want to buy something only 10% faster!).
I'm not calling you a fanboy, but people will do if you can't back that up with some more detail. The 7970 delivers GTX670 performance at a £35 price premium. The 7870 is SLIGHTLY faster than the GTX660 but also more expensive.
7970 is now equal to the 680 with the 12.11 drivers and the 7970 Ghz edition is even faster.
So I would say get a 7970 and you will be gaming on highest settings for a few years
Check the 7970 X2 review. Admittedly a limited data set, but you're still looking at ~5% gain with the GTX680 across the board. Radeon wins the first few, GeForce wins the others. The GHz edition is indeed faster still, but again by around 5%. The differences we're talking about are negligible anyway - 5% is the difference between 40fps and 42fps. It's hard to justify the added cost of a 7970, GTX680 or 7970 GHz over a GTX670 when the difference is costing you at least £35 for the 7970 and even more for the other two.
Also, THG suspect the performance increase comes at the cost of image quality. I'm not so sure (AMD wouldn't want to make that mistake again or they'll start to get a reputation for it) but THG have an article coming up to examine how the gains were achieved, so maybe wait for that. Even if corners have been cut though, no noticeable quality difference could be seen so well-played to AMD for that.