That depends on the scene. Some small scenes with very simple lighting I can build with less than 16GB RAM. Huge scenes that have a ton of lights and geometry will take +200GB Ram (I have 64 and the rest goes to pagefile).
To work regularly with heavy scenes you might want to look at a better CPU and more RAM.
As long as you provide enough room for a big pagefile you could get a lot done with your system though, it just won’t be as fast as a 6/8/10 core i7 with 32-64GB ram of course.
You should however get a better impression of the complexity of the scenes you want to work with so you’ll have a much better idea of what you actually need. For real time visualisations your GPU should be the first upgrade to consider.
To add to Housekillers answer.
Building lighting is entirely CPU, not gpu.
So CPU and RAM are the most important for building lighting.
IT also depends on the amount of meshes and their lightmaps how long rebuilding light will take.
a mesh with a 1024 pixel lightmap will take waaaaaaaaay longer to be lightmapped than a mesh with a 32 pixel light map.