let's say that I'm using a board that has 1 pcie2.0x16 and 1 pcie2.0x4 with xfire support. Should I use both slots with 2 3.0 cards or just one gpu in the x16 slot? What is the best pcie setup in this case to maximize performance?
I would go for 1 powerful card in the X16 slot, the X4 slot will work with crossfire but not recommended. There will be little to no performance penalty (1-2% max) using a version 3 card in ver 2 slot.