There is a difference between 128MB and 256MB cards now. And has been for a while. in ut2003, the map face 3 at highest settings, you get over 40 more fps with a 9800 pro 256MB then a 128MB, and in farcry, it has an option to use extra vram to make super high quality textures that just wont fit in a 128MB card.
And i can bet money that you will see a difference between 256MB and 128MB in half life 2 and doom 3.