Game-Ready question

I Think must ::
- Be import easily into game engine , I think FBX format done this job
- Have a UV Map & Texture ...

You must take a look at the poly's Not more than 50000 polys for the model.

But I read somewhere that textures has to be .tga? Is that true or it's irrelevant?

a low count of polygons and quad mesh is better

Image format is irrelevant, game-ready models must have low polygon geometry, so usually many details are "baked" into textures

Game ready model means both low poly and efficient uv mapping and textures.

With poly count, that's down to the engine you are using and the type of model you are making eg - character, environment, vehicle prop etc. In general the lower poly all of your assets the less power needed to process the geometry which will make it more playable on lower end machines. In most cases for characters weapons vehicles etc they are rigged and animated and more polys may be needed to smooth of the mesh around the joints which will save tearing.

The other big part to the model is the texture and your uv map. Make good use of your uv to get as much detail on your model as possible and don't forget normal maps and spec maps will bring the model to life. The bigger the texture the more power that is needed and graphics memory, so make your textures as small as you can for low priority and smaller models and ramp up the size for more detailed stuff like characters etc.

Just a question that has haunted me for years :) : what interval of polygons can we consider an object is low-poly ?

Low poly is down to the engine being used, as a car is measured by its engine speed and capacity.

In most new games such cod, gears of war etc the weapons are a few thousand polys and the characters are generally 8-12 thousand polys. The lower poly the better but some games place more emphasis on characters such as Mass Effect where Shepard is around 20k polys i think. This is now being stretched a bit bit with the introduction of the new games consoles but will only stretch a bit. Some games like Star Citizen which is in development, some of the ships are 1 million polys plus, they are using a custom cry-engine.

With polys its always lower is better and texture detail is key to the models appearance.

Thanks for the explanation shaun, interesting input.