The 1140 grid fits perfectly into a 1280 monitor. On smaller monitors it becomes fluid and adapts to the width of the browser. Beyond a certain point it uses media queries to serve up a mobile version, which essentially stacks all the columns on top of each other so the flow of information still makes sense. Scrap 1024! Design once at 1140 for 1280, and with very little extra work, it will adapt itself to work on just about any monitor, even mobile.
Download a Photoshop template of the grid at 1140px here. It’s based on how it’s displayed in WebKit, so it’s actually 1133px, as all browsers round sub-pixels slightly differently.
Read more about the 1140px grid.