Crossfire x16/x4?

Do x8 x8. The losses for x4 are much greater than x8

Is that possible with the ASUS M4A785TD-V EVO?

^READ THE MANUAL OR TRY THE WEBSITE FOOL! AND I AINT GETTING ON NO 'PLANE!

Significantly slower. In fact mixing slots like that you would end up with worse performance than using two 8x PCI-E 1.0 slots (which have the same bandwidth as 4x PCI-E 2.0 slots). Of course if you have a 16x and 4x PCI-E 1.0 slot the results are a little worse. Anyway, the crossfire drivers really don't like uneven PCI-E bandwidth so I wouldn't recommend it. You should take a look at this article on legion hardware

http://www.legionhardware.com/document.php?id=761

And you search you can find similar articles on other sites.

That board only has 1 x16 slot, meaning you will take a substantial perfomance hit if you decide to cross-fire.....