I wonder if I create bump mapping for my terrain is it good to add detail mapping too? Does such mix improve realism or make it even worse (because there is "too much" of it)? If this improve realism, is it even worth doing that? Maybe it takes a lot of resources for small improvement?
If you plan to make good looking Terrain, then using Detail Maps will improve your scene a lot, so to answer your first question, yes.
Although only one question should be asked, I will go on. I don't think you should be worried about performance just yet, you will have to try out a few things to find out. Depending on the type of terrain, different games have different solutions that fit them that are balanced between use of resources and enhancing the scene.
Some games might do just fine using a few pretty bigger textures (probably not), some will look ugly and blurry unless they use a combination multiple blended textures, mip-maps, bump or normal maps, detail maps or even decals.
You can either test for yourself, or if you need more advice look at some more terrain tutorials. Wolfire games have a really good Youtube channel where they tell you what technique and why they used it.