Yes i agree with sbinney. You might see a decrease in FPS if you ran them in crossfire. If i remeber right they run as your slowest card. Which in turn would be the card that is hooked up to the x4. Assuming you use the same cards 5770x2 what you would get is they both would run x4 which would make x8 total. As apposed to your 1 card that runs x16 already. So all in all it would be a performance loss. I would also double check ati's website on if you can actually run it in x4. Most video cards are x8+.