How much is 1.0 in pixel?

If I have a screen of 800x600 and
e.g I draw glVertex3f(1.0f, 1.0f, 1.0f)
How can I find out the corresponding pixel values of the coordinates?

I guess I found some answers in the Wiki:
http://www.opengl.org/wiki/index.php/Vie…e_coordinate.3F