That depends on your target monitor size. You should look for the lower common denominator. If you think you will get a lot of small screens, then 955px is a good size to start with (target size would be 1024), but if you think your site will be viewed in larger displays, you can go as high as 1140px (targeting 1280 screens).
It really depends on who you plan to target. There is no one answer for this.
Also, keep in mind, that with the advent of responsive design, then you have to design different layouts, for different resolutions.