What is the memory usage overhead for a 64-bit application?

It depends on the programming style (and on the language, but you are referring to C).

• If you work a lot with pointers (or you have a lot of references in some languages), RAM consumption goes up.
• If you use a lot of data with fixed size, such as double or int32_t, RAM consumption does not go up.
• For types like int or long, it depends on the architecture; there may be differences between Linux and Windows. Here you see the alternatives you have. In short, Windows uses LLP64, meaning that long long and pointers are 64 bit, while Linux uses LP64, where longis 64 bit as well. Other architectures might make int or even short 64 bit as well, but these are quite uncommon.
• float and double should remain the same in size in all cases.

So you see it strongly depends on the usage of the data types.