I know that crossfiring any other HD 5000 series like the 5750, 5850, and 5870 would be pointless because those cards are either too low or too high in comparison with hd 4870's power
But according to reviews, an hd 5770 is slightly slower than the hd 4870..maybe even a little on par? what happens if i crossfire these two? would i be able to play dx11 games? what happens when you crossfire a dx 10 card with a dx 11 card with respect to playing dx11 games?
I'm not exactly sure how the cross-generation CrossFire capabilities work, but I can tell you it will take the lowest common denominator specs and use those. That means if it is possible you will be getting a bit less than 4870 crossfire performance roughly (5770 scales better so don't expect to get that kind of performance; there's overhead that decreases performance when the cards are heterogeneous) and your featureset will be limited to DirectX 10.1 as both cards have to support the same standard.