I don't think so. Most people don't want massive screens and don't know how to effectively use more than one anyway.
Game developers are already having problems with the expense and time required to push out the artwork for today's games. There will come a point, and it might even be here with games like Spore, where the industry will focus more on gameplay than having the most advanced games possible. Regardless, there are quite a few laptops that can play Crysis without problem. I linked to one earlier, and it costs under $1300.
/And/ with more people playing games on consoles, the need for high end PCs just to play games is less important. I think there are many factors that are coming together to make desktops less useful.
Of course there will always be those hardcore fanatics that just need the speed that only a big desktop tower can push out, but those numbers are decreasing by the month I bet.
http://www.pibmug.com/files/wideview.jpg
That people will want ever more realistic graphics, with ever more realistic physics engines?