I need to change my PSU to 750 watt.
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.