Technically, a template can be as wide as you want it to be. The reason many templates limit their widths is due to browser concerns – not everyone runs their browser in full-screen mode, and most people still browse at an 800×600 screen resolution. Horizontal scrolling is evil (generally speaking), so many templates that use a fixed width are no more than about 770 pixels wide.
On the other hand, if you use a fluid-width style, your content width will adjust automatically to browser window width.
In any case, as far as the Kubrick template goes (the default WP1.5 template), you can adjust the “widecolumn” class to be wider (up to about 600 pixels I think) without ill effects. The “narrowcolumn” class can maybe go to 470 or 480, but it is limited by the overall width of the template and the fact that there is a sidebar on pages that use the “narrowcolumn” class.