I don't think you can control this through JavaScript. IE7 has a setting in tools->Internet Options->General->Change how webpages are displayed in tabs->When a popup is encountered. If you have this set to "Always open a pop-up in a new tab" wondow.open() cannot create a new instance of the browser and the window features are ignored. By changing the setting to "Let Internet Explorer decide how pop-ups should be opened", you will get the behavior you are looking for. The control is in the hands of the end user now.
By default, the open method creates a window that has a default width and height and the standard menu, toolbar, and other features of Internet Explorer. You can alter this set of features by using the parameter. This parameter is a string consisting of one or more feature settings.
"resizable" can be set to yes or no to indicate whether or not the new window can be resized. Note that this is a recommendation only as many browsers do not allow you to disable resizing of windows. Setting this to no will disable tabs in IE7.
Instead of opening a new window you could create a limpet object which gets loaded over the top of your other html using the css z-index attribute the same way a sucker-fish menu works -