Well, Blender use processor, ram and video like this:
processor => render images
memory => hold more info of the meshs to use less HD
video => in mesh edition and helps in the render too.

My guess, improve your memory to 8k and if you can, your video card.
Your processor is great

In Blender, you only can edit a mesh of this weight with the sculpt tools.....

I don't think this is a good practice, because in blender, when yoy use the Multi-Resolution modifier, you can make models like in Z-Brush, but with much less memory to work on it, and make easier to generate the normal maps of your model.

Take some tutorials, and try this in the future.

Well, I select the High poly, and the lod0 (in this order), and use the Bake.
Normals - Tangent. Margin 32, Split-Fixed.

LOD 0 is the portraid model, LOD 1 the game model.

Well, My method is totally the opposite, I model the Lod 0 with care, map this with care, and after this, I create a copy and increase the edges count in hard corners, and use the modifier "Multi-Resolution". With this, I use the sculpt, and map the normal map.

well, I don't know what method is the best. But I think reading this one, will give u some ideas.

Thi is a sword that I make with this method.
http://i.imgur.com/kG88CWP.png?1

I can't open the link. But I'wll tell u how I do this kind of thing.

1- model a low poly model, with +- the shape that you like to model.
2- use the mult surface modifier to increase the model polygons, and sculpt this, increase again and sculpt more. Don't pass 5 subdivisions in this modifier, to safe your performance.

With this method, you can extract all hero content.....

If you want to model a hero, extract the files from the game.
I don't have the information here, "how extract", but if you search, you will find it easy.
This is the right way to make a item, because the downloadable model, sometimes have diferences of scale, or bone/ vertex names..... I had this problem with my first set.

Do you recalculate the normals of the High and Low poly?

X, in game engines, is what the engine shows when some particle have a bug.... and a square when a model have a bug.....

Well, this is what I know, how fix is another history that I don't have minimal idea =).

lumiére, that 1 px texture, is for little detail and mesh, that you don't want to like transparent from behind.
http://www.dota2.com.br/threads/refazendo-set-strength-of-the-futures-gods.20434/
Like the shoulder bottom part, the back side is all black.

I'm working on this set, then don't look to much to the textures.