the only difference between 32 and 64 MB of video memory is the amount of texture bitmaps an application can load into it.
Up to now the standard is 32MB and most of the games out there use only this 32MB. When 64MB will come more popular this will change.
With "only" 32MB an application can already use more texture data than would fit on it by using the AGP feature -> textures are located in system memory and accesed by the gfx chip via the agp port.
The only problem is, that the agp memory access is slower then the onboard video memory.
Hope that helped