>>If you want to use your PC with two monitors, how important is to have both of the same size? The reason I am asking is I have now one monitor (23") and want to add another one. But I don't have much room on my desk for another 23" so I am thinking of getting a 19" monitor as the second one. Will it be a problem?
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>Hrm... I wonder if it's possible to have one monitor in landscape orientation and another in portrait orientation...

My old nVidia 6500 something, from, IIRC, 2003 (the last good one that didn't need it's own fan (club)), has that option. The GUI for setting it up is a bit labyrinthine the first time, but nowadays, on its fourth motherboard and about sixth installation of Windows, it's become almost simple.