Here are some before/after screenshots with two shadow settings so you can estimate what settings might be suitable for your machine.
iShadowMapResolution 512 vs 4096
As you can clearly see the shadows on the high resolution pictures are a lot sharper and look much better.
iShadowMapResolution 512 vs 4096
512: If you look at the shadows further away, they look “ok”, but the shadows close to you have a lot of rough edges
4096:
