Windows 7 Home Premium is available in both a 32-bit model and a 64-bit model. Although both types of Windows 7 look and act the same on the surface, the way they work is quite different. How do you choose between the 32-bit and 64-bit versions? It really boils down to your needs and capabilities.
The 32-bit flavor of Windows has a limit on the amount of memory that Windows can use. It really can only see 3.4 or 3.5 gigabytes (GB) of memory. Even if you put 8GB of memory into your computer, the 32-bit version of Windows can only use about 3.5GB. The rest just sits there, wasted.
On the other hand, the 64-bit flavor of Windows 7 can use much more than 8GB of RAM. Whether you need that much additional memory is debatable, but some power users will definitely want the extra memory.