Once I have the image the way I want it, I resize, setting the longest side to 600 pixels. Then I do Save for Web and set the quality so that the filesize is as close to 150K as is possible.
i guess that would depend on the photo... for instance my abstract had to be close ebough to a square, so i had 640 x 600 or so.... but it is usually nicer to have a rectangular shape to it, just because traditional photos are of that dimension.
may as well just make it 640 pix on the longest side....
I sometimes sacrifice a little size (like to 600) so that I can save it at a higher quality setting while still keeping it under the 150k limit.
And I personally find the 640x480 video-based frame the least appealing of the "standard" shapes -- I myself prefer 640x427 (3:2 like 35mm), panoramic, or square.
