WHEN DO I NEED MORE VRAM?
Is there any situation where you could put the 256MB of GDDR3 SDRAM to work? I suppose if you are running your MacBook Pro in dual display mode with an external 30" Cinema display, which assigns 128MB to each display, it could help. And if you are running a video memory hungry pro application like Motion 3, it could help. We're still trying to find a way to quantify this.
It was suggested that we test the 3D games at "Ultra" Quality in Doom 3 and Quake 4 to see if that creates a bigger gap between the 128M and 256M models. We also plan to try enabling 4X FSAA to see what impact that has.