Preprocessor to use a different template for popup window
You probably want:

print render($page['content']);

You can find the variables available to you in the template by printing get_defined_vars() using print_r(), core's debug() function, or one of the extremely handy devel module debugging functions such as kpr() or dpm().

I'd install devel and do this:

kpr(get_defined_vars());

Your modal method might not play nicely with page cache so I'd definitely test to make sure it fits your usecase.