According to Microsoft,  if you want to run Windows 7 on your PC, here’s what it takes:

• 1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor

• 1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)

• 16 GB available hard disk space (32-bit) or 20 GB (64-bit)

• DirectX 9 graphics device with WDDM 1.0 or higher driver

However, these requirements are the minimum requirements.  The requirements for a better experience is:

• 2 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor

• 2 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
  (Preferably 4 gigabyte GB of RAM

• 16 GB available hard disk space (32-bit) or 20 GB (64-bit)

• DirectX 9 graphics device with WDDM 1.0 or higher driver