The bigger the monitor, the better card you will need. The HD5770 will max everything on 1280x1024 (19/20'' square monitor), or 1440x900 (19-20'' widescreen monitor''). You won't be able to max everything on a 1680x1050 (22-23''), and definitely not 1920x1080 (23-25'').
But if i buy 19Inchi LCD what benefit did i get when play extreme game like Crysis 1...?
While I don't want to stifle your enthusiasm, it's probably not a good idea to upgrade your card just for one demanding game. You should still see performance gains from running a second 5770. It most certainly won't be a doubling of power though and at lower resolutions the benefit of crossfire (or 2GPUs in general) is typically much less.