i did a quick test...

32bit blender with a 100000 poly mesh takes 75 megabytes
64bit blender with a 100000 poly mesh takes 95 megabytes

32bit blender with a 400000 poly mesh takes 230 megabytes
64bit blender with a 400000 poly mesh takes 295 megabytes

i wouldn't have thought that but the difference still isn't 40% but about 25%. don't know how it scales though...