You can use up to 3GB in 1920 1080, but you max out absoultly everything. Like GTV IV make depth of field the highest, AA highest, and texture detail highest and you can get close to using 3GB. But for practical use, 3GB isn't that necessary unless its cheap. I have MSI 580 Lightning, which is factory overclocked and after market coolers that I would recommend over a 3GB 580.
GTA IV is also probably the only game that does that too :P