Game object modelling general practices question

Hi, I have just started to use blender and am wondering about the general process to make a game object.

Firstly, should the game objects be made of multiple objects?

http://img5.imageshack.us/img5/3148/k79j.png

Left is made out of more vertices because the rectangular shape thing is extruded out of the object while the right model is made of less vertices because it is just made out of two objects. It isn’t really a good example but the idea is there. As more and more details are added, the object would have exponentially have more vertices if the left method is used compared to the right. What are the pros and cons of these two types of modelling?

Secondly, are there any guidelines that I should follow when modelling for games?

Lastly, are there any websites that provide free blender models(mainly low poly/medium poly for games)? I have no intention in copying anyone’s work but I do want to study the mesh of the objects and gain some insight on how certain models are made.

Thank you for any help! Cheers!