System requirements for 5 man multibox @ 4K

[MiRai]

I'm interested in starting to run a 5 man in WoW, but would also like to upgrade to 4K graphics.

I multibox at 4K on a 6950X and a 1080Ti, and it comes down to this: Upgrade your GPU and see how performance is. If it's good, then you don't do anything else, but if it's bad, then you can upgrade the motherboard and CPU too.

2 cards would require a new mobo as well, as my mobo can’t do 2x x16.

There is little to be gained from SLI in windowed mode these days. However, has there been some new evidence that shows x16 PCIe is better than x8 PCIe in terms of modern GPUs? Every time I see the question asked about x16 vs x8 there is no evidence to show that modern GPUs can completely saturate even just 8 PCIe lanes.