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You can't CF a 6950 @ 6970 with a 6950, the PC will see the modded card as a 6970. Has to be 2 of the same cards, brand doesn't matter but they both have to be either 6950s or 6970s, you can cf 2 modded ones though.
Also you'll be fine with 2x6950s unless you are going for big resolutions, 25600x1600 or eyefinity etc. Even then they do a stellar job.
It works to whatever your slowest card is, so no you wont have to adjust the speed.
You can't CF a 6950 @ 6970 with a 6950, the PC will see the modded card as a 6970. Has to be 2 of the same cards, brand doesn't matter but they both have to be either 6950s or 6970s, you can cf 2 modded ones though.
i dont think that is right actually, i think you can crossfire any two cards al long as they have the same first two digits
69xx and 69xx should work
One bridge is how it should be.