I want to use a grid system, like say grid 960.
But I want the top header of the website to be 100%, is this still possible?
Yes, it's possible.
The first step is to add this code to your CSS:
body,canvas { border:0; margin:0; padding:0; }
Then make a class:
.full-width-div { width: 100%; }
And then just add it to your HTML like this:
Your content...