Marshall - 12:46 pm on Jan 31, 2013 (gmt 0)
The ideal width is all of them. The magic word is Responsive.
Not necessarily true depending on the nature of the site. While responsive designs are ideal, a maximum width is a good idea. If you have a site with a lot of text, like this one, a blog or similar, a paragraph becomes a very, very, very long sentence on wider screens and difficult to read, not to mention it looks rather stupid. Personally, I max out main content areas at 800px which is still a good guideline.