Many of the top websites (including this one!) work with fixed widths, centred.
This is done to maintain the look and feel of the site, as proportional sizing can be tricky, and not end up looking like you intend.
These days it is common to design for 800 pixel wide screens, and more and more common to design for 1028 pixel wide screens.
You can implement statistics (sometimes your ISP will do them) which will tell you the resolutions at which people browse your site (and a whole lot more too), which would help you.