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If you want to CF 5770 who give a **** ABOUT 16 PCI lane? The second card on the 8X wont even be bottlenecked. So P55 is still perfect. Or if you absolutely want to have 2 then get a 5850 and OC the hell out of it it will be a better solution.
I would also concider Asus P7P55D and EVGA P55 like this one.
Newegg.com - EVGA P55 SLI 132-LF-E655-KR LGA1156 Intel P55 ATX Intel Motherboard
They actually are better then MSI.