OpenGL is good in that you can scale the coordinates. For example if you use Ortho style drawing, you can do something like:
Code:
glLoadIdentity;
glScalef(ClientWidth/100, ClientHeight/100, 1);
And now your coordinates will be 100 x 100 regardless of window size. Any graphics drawn with these will at all times be drawn in full window area, and you don't need to care if it's actually 1024x768 or 640x480. But often times you don't want to do that, because fonts or certain graphics will bend and look bad. Still, it's much faster than drawing to a buffer and scale it.