i will defently not buy an 6950 if you allready have an 6870. the performance difference will not be worth the money.
will.... unless you are willing to bios unlock it. then it might be.
here is a link:
AMD Radeon HD 6950 to HD 6970 Mod | techPowerUp
thats what is so nice about the 6950. but if you are not into all this overclocking and tweaking, then i will rather go for crossfire.
so if i where you i will go for crossfire 6870 or buy a 6950 and bios unlock it to a 6970.
will... if i where you i will go for an gtx 570.. but.... i dont think that thats what you want.