Reactor Worker explained this to me when I was buying his GTX260, which has 768 megs of video memory.
Basically, if I remember him correctly, the difference between 768MB and 1GB is only a huge issue if you're using high resolution textures. Edit: like Otis said, higher resolutions.
I know you should never trust the person that's selling you something for accurate information, but RW seems like a pretty honest, knowledgeable guy.