Windows x64 operating systems are 64-bit operating systems!
The 64Bit allows to address much larger amounts of memory linearly! With x86 (x32) you can only address up to 3.5 GB of RAM under Windows. At x64 theoretically up to 16 TB of memory. But most motherboards do not usually support this amount of memory and because of the operating systems themselves there are also limitations on how much memory is managed!
For example: Windows 7 Starter can only manage 2 gigabits Windows 7 Home Basic with a maximum of 4 GB of RAM in the 32-bit version. The 64-bit / x64 version supports 8 GB!
For Windows 7 Home Premium, the memory limit is 16 GB.
Windows 7 Professional / Windows 7 Ultimate / Windows 7 Enterprise is 192 GB!
Windows 10/8.1 HOME 16 GB
Windows 10/8.1 Professional / Enterprise is 256 GB!