I'm thinking of upgrading my Unraid box and want the ability to support x2 gaming VMs - I was thinking about either the 2700x or the 9900k as they are both 8 core/16 thread.  I saw the 2700x only has 20 PCI express lanes and the 9900k only has 16.  I also have a few expansion cards (x8 SAS card, x1 Network card) and wanted to drop in x2 GPUs (1070 and 1060).  Would I even have to option to support x2 GPUs (1070 & 1060) plus the x8 and x1 cards with either of these CPUs?  I've never really understood if PCI expansion slots are determined by the CPU or the MB?  

23 minutes ago, russdyer77 said:

Would I even have to option to support x2 GPUs (1070 & 1060) plus the x8 and x1 cards with either of these CPUs?  I've never really understood if PCI expansion slots are determined by the CPU or the MB?  

Yes, though with limited bandwidth, I'll speak just about Intel since it's the one I'm more familiar with, you have 16 CPU lanes plus the PCH lanes, depending on the board there's usually one x4 electrical slot plus a few more x1 slots, so you could use the HBA on the x4 slot, though you need to mind NVMe devices, etc, since most times it can't be all enable at the same time.

So just to clarify as a general principle - if using a 9900k & I purchased a ROG MAXIMUS XI HERO mb as an example:

 

the top two x16 slots would use reduced bandwidth (from 16 to 8 ) if I populated with two GPUs & the remaining SAS and networking cards would run off the mb's lanes?  I don't plan to run NVME drives, so this probably wouldn't be a concern.  But if I'm concerned about getting GPUs to use full bandwidth, I may need to consider the Intel X or Threadripper lines?  

Correct, that's how all SLI setups work, instead of one x16 slot you get two x8 slots, and that's more than enough bandwidth for most (all?) current GPUs.