Building ontop of my current setup: Crossfire PCI-e speed question?

[kukko3]

So, I'm going to be gearing my current rig up for mining for the summer. Looking at hardware options I've decided upon a a GPU upgrade for now. I'm currently running an ASUS M4A77TD Pro motherboard, which for crossfire only supports 16x/4x crossfire. So my question is, how much will the 4x slot throttle my performance in mining if I go for a crossfire setup? If the impact on performance is not too grave, I'm looking to go for a 2x5870. Alternatively I guess I'm also willing to invest in a single 6990. Thanks!

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[fasti]

If it's pci-e 2.0+, you can even game with that crossfire setup and notice no changes to x16 x16 setup :P

But 2x cards on one motherboard is a pain in the butt(so much heat if you won't use extenders). I would rather get dual gpu card.