Multi GPU Index
The Hardware Asylum Multi-GPU index is a portable set of data we can use to determine the multi-GPU potential of any motherboard. A higher index number indicates a favorable condition for multiple graphics cards while a lower number indicates a hindrance in one of our leading indicators. Since the Index is not dependent on system architecture it can be applied to any motherboard regardless of processor or chipset generations.
Max Support: 3x
This motherboard will support up to three graphics cards and is optimized for dual card operation.
For best video performance you will want to populate the top most PCI Express slot with your video card and try to avoid using the secondary slot. The secondary slot is hardwired for 8x PCI Express lanes and borrows from slot 2.
Boards like this are really quite fun to mess with and even though the last slot is hardwired to 4x lanes you can still run a third card either as a dedicated PhysX processor or play with 3-way Crossfire. The best option would be to reserve it for hardware RAID controllers or leave it open for better dual card cooling.
By the looks of it this layout is pretty common with most of the Z490 motherboards from ASUS. There are a few exceptions once you get into the halo product region.