Current card does not saturate PCIE 3.0, 2080 super is close (2080 super 15.5GB/s, PCIE 3/16x does 15.75 GB/s ). Nvidia Ampere should though.
The difference is actually negligible for gaming.
https://www.gamersnexus.net/guides/2488-pci-e-3-x8-vs-x16-performance-impact-on-gpus
Found a test with 2060 supers:
PCIE doesn’t matter much for rendering either. The scene will mostly be loaded in the GPU at the start then rendered with minimal PCIE use.
Now, for a 3950x, I’m not even sure there is a motherboard without PCIE 4.0 available. If you want to go expensive, you could even get a PCIE splitter from one 16 to 4x4. But as silex said, don’t skip on the PSU and the cooling.
SLI is useless for blender if I remember.