I try to keep mine at 400 or just a bit below. That's one reason I usually end up with double images...one I resize to use on my page and in comments and one of the original size. Of course, I'm never sure how it all looks as so many of us use different size monitors. What fits perfectly on one has to be scrolled for another.
One thing I have learned to do when I'm working on my page...make sure my compatibility button in on for the site. Otherwise, centering makes no difference whatsoever, and the page just doesn't look right to me.