Re-size them after you take the pics and download them on your computer. Don't cut it down on the cameras as that will take some of what you want away. So take the shot with all your mega pixels and then crop and re-size after you crop them.
You can even re-size them at photobucket. I do mine on my computer with photo software from Roxio. One thing I like about that software is I change it to tell the pixels and then I just change the first one to 640 and hit enter key and it will auto to the other side.
That is just my opinion though.
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