16x4x Crossfire doesn't perform as well as 8x8x and is nowhere near 16x16x. However, two 5770's would not saturate an 8x8x lane. Unfortunately, you've got a 4x slot. Best advice: get a faster card or a board that supports at least 8x8x. 16x16x you could not make use of given two 5770s.
Putting current graphics cards in a PCI-Express x4 slot basically verges on madness and incurs a performance hit of between 25 and 33 percent. Even if the motherboard in question were especially cheap, the card’s price/performance ratio is affected immensely. When using an x8 connection, performance also decreases, albeit only by 7 to 8 percent.
From the conclusion.
Crossfire meets PCI Express 2.0 â€“ More Lanes, More Frames? : Crossfire up to 20 Percent Faster