I won't consider only the desktop monitor resolutions. Nowadays you can browse websites on almost any device, and is fairly common for people to browse also with their mobile phones and tablet PC, so you really should consider them both.
This could result in a different site version for different devices, or a site layout that is ableto accomodate based on different sizes. This is something you have to consider when first thinking about the website you are planning to do, since not every website is suited to have a layout of this type.
An adaptable layout has the added benefits that it will be good even if new devices with custom (non standard) resolutions came out, you won't have to worry about checking it in those new devices because it will fit ok (if the layout was done right).
Anyway my standard minimun width is still 960px for desktop monitors, and for an adaptable layout I usually choose a max-width for really huge monitors (think about viewing a site which extends for the entire monitor width on a 2500x1600 monitor... gosh) of about 1200px, and a 100% width if the browser size is less than that, to allow the website to fit good everywhere.