What I'm thinking is that maybe it's better to get the MSI 6950 with much better cooling so I can overclock it better, then the PNY gtx 570 with the regular cooling. In any case I want the option to add a second card whenever the computer will be too weak.
Looking at those prices, I too vote for the MSI 6950 2GB card. It is overall an excellent package and can practically handle every game available now (and to come) with very high detail levels. Also, it can be very nicely overclocked to attain higher fps numbers. Crossfiring is great too.
The HD 6950 2GB is IMO a better card currently when compared to the GTX 560 Ti. Surely some games favor one card or the other and there is where you will see major differences. Take civilization V for example. It heavily favors Nvidia cards for some reason. So if that is one game you play heavily then surely go for a Nvidia card. Else both are good.
Coming back to specific cards, both the 560 Ti and 6950 are very much overclockable. All you require is a good cooler aboard. The MSI Twin Frozr series are well established overclocking performers. And being the least prices from the lot, it easily gains the recommendation (once again).