Wah long time no see.
Z97 if you are running on 980ti sli, then you probably out of pcie lanes to run on your new nvme m.2 drive that requires x4 lanes.
In all haswell has only 16 pcie lanes. X8x8 has been allocated to your 980tis.
Unless it is a amd xfire card, it will then run at x8x4x4.
X99 depends on the cpu you buying. An i7-5820k has a total of 28 pcie lanes. It is more than enough for 2 gpus with a nvme m.2.