Size of the monitor and performance.
"size" or "resolution"?
If we are looking for a card that will last a while, and 1GB is already becoming a limitation at 1080p in some games(a trend that will only continue if history is any indication), then getting a 2GB card seems like a good idea. The card does need to have a gpu powerful enough to take advantage of it, though.
I'm thinking it would be worth it to spring for a 7850 2GB(you can overclock the snot out of a card like that).