Honestly, no one knows. It's all guessing and overkill in the high end servers. EE used to run with 96GB ram and 2 hex core processors, and it still wasn't enough.
The game changes so often and the community is constantly pushing it. Bottlenecks that cause lag can be anything from someone landing on a planet to someone simply logging in or out.
RAM usage depends on the size and number of entities being loaded in. same for CPU, except you need to account for physics like idiots in kilonaughts de-orbiting planets.
You'll never get a solid estimation that's accurate at this stage of the game, especially in a sandbox that lets you build from a single block ship all the way up to several kilometers. It's like asking what the minimum specs for the client are. It's based on what you are going to do.