The screen resolution is of minimal concern when looking at memory on high-end dx9 cards. The memory is used primarily for textures and can improve performance during gameplay be maintaining the textures in the video cards ddr memory (not vram anymore) rather than fetching it from system ram or hard disk.
The real question for people considering a 256MB card is this: What titles do you play that will actually utilize the extra memory? If you are building a machine which will be used exclusively for Half Life 2 and Doom 3, then you will see some benefits from having more ram. If you don't use extensive 3D, then 64MB or 128MB will be more than sufficient.