I've noticed in Afterburner that not many games/apps (least that I have run so far) use more than 1gig of video memory. 3dMark11 only uses about 400Mb max, left4dead2 uses about 800 which is a common point for a few of them. Crysis2 has used the most so far, on full settings it uses 1.4G. I game at 1920x1200 so I find it interesting that cards like mine with 3Gig are really only useful for multimonitor setups or if you got like a Dell 30" monitor (yeah I know other ppl make 30s). Just an interesting factoid. If you strictly game at 1080p then 2gig is way sufficient.
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Nowadays gaming 2gb is enough.but in coming next year game will be higher demanding so in future game will be using above 2gb at 1080p or more.
From what i have read, the extra ram really comes in useful if you add a higher Anti-aliasing. Crysis 2 recommends 1.5 GB of Ram for VGA if you run high resolution textures and Dx11 at the same time.