if a game uses more than your video cards vram, you can still run it. Although then it spills over into much slower system memory which slows things down significantly. The biggest user of vram is textures, so lowering your textures down from the maximum will help, and you can still keep other details up high. AA and high resolution displays can also increase vram useage quite a bit. If your buying a high end card like a 680, 770 or 780, I would recommend the 4gb models for a little future proofing. New consoles will have a lot of ram, so you can expect large, high res textures to be in upcoming games.