Is there any reason to upgrade my AMD Ryzen 9 5950x to a 7900x3d or 7950x3d?
I'm currently using an AMD 5950x and an Nvidia GeForce RTX 3080 LHR, pushing video to a Dell U4021QW, which is a 40" monitor running 5120x2160@60hz. A few of the games I play are pretty challenged at that resolution, so I'm thinking of upgrading at least the video card. Is there any reason to build a new system with a newer CPU as well?
I'm not sure what you mean by "challenged." A drop from 60fps to 55fps? No fps loss, but high GPU temps?
You might want to look at the 5800X3D before worrying about an entirely different mobo, ram, and processor. If you're not doing 3D rendering or other multi-threaded intensive work, it might be the better option over your 5950X, and you could save yourself some money.
But if money is no object (and I wonder if you'd be posting here if it wasn't), then upgrading to any of the X3D chips would probably be a fine upgrade for many years.