Hi, I'm currently planning to build a system which can support a dual Radeon HD4870 crossfire setup, and am currently divided between the 790FX (Gigabyte GA MA790FXT-UD5P) or 790GX (MSI 790GX-G65) motherboards.
I know for a fact that the 790FX board supports PCI-E x16/x16 dual crossfire mode while the 790GX board only supports x8/x8 so the question is will the performance of a dual Radeon HD4870 be hampered on the 790GX board and if so, by how much? I haven't been able to find a solid answer for this question so far.
I'm also aware that a Radeon 4870 X2 card (dual GPU) currently works on a PCI-E x16, does this imply that a single GPU 4870 is already at its best performance on a PCI-E x8 connection?
I suggest you take a look at the product specs of the card you're planning to use. If the HD 4870 is listed as working on an x16, then make sure you have two x16 slots for both cards. Concentrate on any potential bottlenecks for both cards.