Don't forget that higher gain antenna only means more directional, so get the geometry between Tx and Rx wrong and it will make matters worse. Gain directionality will also be more sensitive to near by metallic objects. Don't know about polarisation either.
From memory, the antenna will have a doughnut shaped pattern with max strength orthogonal to antenna axis (though if I am wrong no doubt someone will chirp up ), so antenna should be perpendicular to line of sight from computer to router.