it's a bit hard to give sound advice without knowing more about what your original footage is and what you want to have in the end... but from a wild guess i think you have full frame anamorph 16:9 PAL footage and you want to letterbox that.
so, as a step by step line out:
to get from PAL footage to square pixel you need a horizontal stretch of 1.067, ie. resize your footage from 720x576 to 768x576.. to get from that to true 16:9 you need another factor of 1.333 (768x576 -> 1024x576) now you have a undistorted picture where you can do your effect work ect. then you want to resize this back to PAL non-square pixels (1024x576 -> 768x432 -> 720x432) ie a horitontal squeeze of 1.422 and a vertical of 1.333.
obviously the much faster way would be to just squeeze the original vertically by a factor of 1.333... however, in some cases i prefer to do my effect work in a 1024x576 master.
setting the pixel aspect in the globals certainly is useful for effects that work on the x and y axis.. for example for your original 16:9 footage you'd use a pixel aspect of 1.422 (1.067 for PAL non square x 1.333 for 16:9), so if you apply an xBlur of 100, the yBlur will be corrected to 70 automatically (100/1.422). but it does not affect your original footage.
the viewer aspect and the proxy scale can help for display (but wont affect renders). however, i tend to avoid them on PAL footage as you dont see the true pixels..
uh, i guess i dont make much sense.
++ christoph ++
