Is it the model or the skin that causes the FPS drop?? I.e. if you install some nice custom model with one kazillion polys and that use the default skin, would it affect your framerate more than using the default low-poly model with a high-res skin??

It's a little bit of both. But usually, i'd say the model will affect more likely than a skin will.

Skin uses up graphics memory.

Polygon count effects FPS as the engine has to render more triangle per pass.

Ok thanks guys!:)

When the textures exceed your graphics card's memory your fps will drop sharply though.