X10D As in you looking at Dual Xeon E5s? I would recommend the X10DRi. 3x PCIe 16 slots.
Since you currently have a single socket board and you are posting on the NK 5.0 thread I’m guessing you are looking for something similar which are the X10S boards. In that case I would recommend the X10SLH-F It has the most current LGA1150 chipset (C226), a reasonable amount of PCIe connectivity for expansion, and IPMI.
However The NK 5.0 / LGA 1150 socket is a bit old at this point. For your upgrade I would actually point you to the NK 6.0 guide on LGA 1151 and 8th / 9th Gen CPUs. You should see a significant performance boost over the NK 5.0 for only a moderate price bump. For an NK6.0 I would recommend the X11SCA-F.
Just for fun, there is a seller on Ebay currently offering used X12SAE at a reasonable price. If you can also find a good deal on a 10th gen Intel CPU you could get ahead of the curve. The caveats though are that 10th gen CPUs will be a significant price increase over 8th / 9th gen CPUs for only a marginal performance bump and you would basically be a guinea pig for what will someday be an NK7.0 once prices come down. If that doesn’t sound like fun to you then probably sick with an NK 6.0.
No matter which choice you make it will be a significant upgrade over your current NK 4.0 setup.