32 bit 1650x1080 /8 = 71,280,000 bytes per screen. And games should use 3 screens for rendering. ~ 203m.
U "should" render screens whilst they`re not being displayed, so that draws aren`t visible to the user. U draw the view off-screen then point the vision to it`s location once it`s all done. The third is blanked as one is drawn.
Many engines use different variations of this tho.
Also a "screen" as initially described isn`t necessarily the size of the viewing window. 1 of them may well be, but the remaining ones possibly not.
U can see where a 256m card can struggle @ 1650x1080. But variants of this system will adopt system memory to do the bank switching and stuff, this can slow things down somewhat.
Some engines could very well get away with using 2 screens in video memory, but then u have to think of the textures`n`stuff - the more the gfxcard can keep in it`s memory and not dip into the system, the faster the end result. (actually these days I guess it doesn`t take too long to blank 70m or so)
I can see the need for a gig, sorry GiB (freakin idiots), of memory. What if u want over 1650x1080 ?