If you're only planning on desktop users viewing your site, 900px could be a little small and you could, easily, go for a screen resolution of 1024px x 768px, or greater with no, or minimal, consequence.
On the other hand, if you're planning on your sites being usable to those surfing on a nettop, or a mobile phone, however, they may require a great deal of scrolling. Which is irritating at the best of times.
That said, you can take advantage of things like @media queries to serve different stylesheets to your users based on their screen size, so a desktop user with an Eyefinity > 3000px display will get a different look to a user surfing via 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 stylesheets to serve whichever is likely, based on screen size, to give the best view to the user.
Personally I'm a fan of the fixed-width-centred layout, which allows ease of designing and allows a certain liquidity. The benefits to this approach, for me, are that long-lines of text are difficult to read (and presumably why A List Apart a variation of that approach in its articles).
I think that if you design a site to be useful to yourself with the window maximised and then at different sizes, you should end up with something that looks good at most. But remember that as screen size gets larger, handling white-space properly becomes a necessity.
It's worth noting that, when you design for a particular horizontal resolution, you do have to take into account the browser's chrome (things like screen borders and scroll-bars. I think that's usually about 40px for the scroll bar, if I remember correctly). This seems to have been the onus behind 960 Grid's use of 960 horizontal pixels, not only is it divisible into columns, but it also allows for the (average) user's browser to have a scroll-bar without causing an overflow and a horizontal scroll-bar as well.