If you only plan on desktop users browsing your site, 900 pixels might be a little small, and you could easily find a screen resolution of 1024 pixels x 768 pixels or more without any or minimal consequences.
On the other hand, if you plan to use your sites to surf on a nettop or mobile phone, they may need a lot of scrolling. Which is annoying in better times.
However, you can take advantage of things like @media queries to serve different stylesheets to your users based on their screen size, so a desktop computer user with an Eyefinity display> 3000 pixels will look different for a user browsing through an iPhone.
As an example:
<link rel="stylesheet" href="ridiculousDesktopScreen.css" media="only screen and (max-device width:3000)"/> <link rel="stylesheet" href="largerDesktopScreen.css" media="only screen and (max-device width:1280)"/> <link rel="stylesheet" href="normalDesktopScreen.css" media="only screen and (max-device width:1024px)"/> <link rel="stylesheet" href="mobileStylesheet.css" media="only screen and (max-device width:480px)"/>
And then use each of these style sheets so that they appear depending on what actually depends on the screen size to give the user the best look.
Personally, I am a fan of the fixed-width layout, which makes it easy to design and allows for some liquidity. The benefits to this approach, for me, are that long lines of text are hard to read (and, apparently, why A List Apart is a variant of this approach in his articles).
I think that if you create a site that is useful for you with a maximum window size and then with a different size, you should get something that looks best. But remember that as the screen size increases, the processing of the content takes up properly.
It is worth noting that when designing for a specific horizontal resolution, you need to consider the Chrome browser (such as screen borders and scrollbars. I think that usually about 40 pixels for the scrollbar, if I remember correctly). This, apparently, was not essential for the 960 Grid using 960 horizontal pixels, not only being divided into columns, but also allowing the (medium) user browser to have a scroll bar without causing overflow and horizontal scroll bar.