I have my second monitor in 1920×1080 portrait and sometimes that is an irritation at 1080px wide long before getting to the 640px your columns have. To pick one example, in Google search pages you lose over half the right-hand column without scrolling (though at least the main results are fully visible). A lot of layouts seem to assume having around 1280 pixels to play with (less a little for scroll bars).

I've found zooming out a little works fine usually, and modern browsers remember the setting, but that isn't perfect. If I unzoom on the main display it does so for tabs open on that site on the second too (to use the Google example again if I open maps or an image search on the main screen I probably want it at 100%) and some sites seem to actively resist being zoomed.