couple of things for display in afx:
check that the display option for the comp is set to full, same for your previews.
also, if you're using vector art make sure the continuos sampling is checked for that layer on the timeline panel, while you're there you might want to check that the antialiasing quality for the layer is set to high.
you might want to make sure that pixel aspect ratio preview is off though this shouldn't affect rendered files.
for renders:
check all your settings in your render queue.
if you're rendering to a compressed file format (DV pal/ntsc) expect quality loss.
if it's a red line in DV expect edge artifacts.
if you're rendering to quicktime make sure the display hi quality switch is set in quicktime (CTRL - J /COMMAND - J)
if all else fails reset afx and turn off open gl.