For resolutions up to 1920x1200 1GB of GDDR is sufficient within every game I have played except for Metro 2033. Metro 2033 uses about 1.2GB on max settings using my GTX580 SLI setup (as measured within Afterbuirner).
For most people gaming at 1920x1200/1080 or lower, 1GB will be absolutely fine. A single 6900 GPU will not be able to handle Metro at max settings at this resolution anyway.
For most people gaming at 1920x1200/1080 or lower, 1GB will be absolutely fine. A single 6900 GPU will not be able to handle Metro at max settings at this resolution anyway.