As far as i know the 1GB 6950 cant be unlocked using a Bios flash to make it a 6970 while the 2GB card can. As you have an SLI board then obviously if you even think you might consider dual cards then it has to be the 560.
The 6870 would be cheaper as its noticeably weaker.
So as i see it your choice comes down to, do you want 2 cards in the future ? Do you want to try to unlock a 2GB 6950 to a 6970 ? Or do you just want the best bang for the buck single card now ?
If the 2GB 6950 didn't unlock to a 6970 for any reason ( I have not heard of one that didn't yet) As this is possible as all hardware is not the same and there is no guarantee, then its of around the same performance level as a 560 both of which can obviously be overclocked. Again no guarantee what speed you could get.
So unless you know you want 2 cards in the future the best bet is to get the 2 GB 6950 and try to unlock it.
I would recommend the Gigabyte GTX560 TI SO (Super Overclocked), it's really bang for buck at the same time it preforms as good as 6970 and GTX570 in many benchmarks @ 1920x1080. It crushes 6950 in most tests i've seen. For sure the card i'll chose for my next build if nothing more bang for buck shows up within LGA2011.
Check the benchmarks of the games you wish to play. Faced with a similar choice a few weeks ago, I chose the GTX560 after seeing how it utterly destroys the ATi cards in Civ V, the game I was most interested in maxing. I could see that for other titles, the HD6950 would have been better. So, check those benchmarks!
Since most 6950's are clocked at reference speeds, and most GTX 560's are sold with factory overclocks, the strategy for making benchmark comparisons should be to compare a particular GTX 560 version to the standard 6950. Like this review, for example, of a moderately overclocked MSI Twin Frozr:
Anyway, don't count on being able to upgrade to Bulldozer without replacing your motherboard. Bulldozer CPUs will need AM3+ boards and will not work on AM3 boards. I think they are adding a few pins to prevent you from even trying. If you're going to upgrade your board in the future anyway then I say go with a 6950 now and add another latter.