There's actually no difference between 32Mb and 64Mb cards on Q3, other 3D games won't get big advantage either. However if you play 2D it will use extra video memory for surfaces.
If you mostly work with 3D... 3D Studio MAX doesn't seem to work faster with +32Mb, but other software may use it for acceleration.
Also I'm not sure what video card you have in mind, if it's MX400 I think you should take 64Mb version because of tiny price difference.