Allyn Saroyan , Apr 30, 2010; 08:39 p.m.
I use "JPEG fit within 1024x1024" for e-mail and "JPEG 50% of original size" for a Web page.
JPEG does reduce the quality of your images as the compression increases. I do not know what JPEG compression setting is used by Aperture. I find, however, that either the 1024 or 50% output looks good to me.