Dell optiplex 9010 Windows 7, 32 to 64 bit OS migration for supporting more than 3GB RAM

Hello All,
I have Dell optiplex 9010 Desktop System with Windows 7 Professional 32 OS and 2 GB Onboard RAM.

I wanted to upgrade the Desktop so ordered 2 units of  8 GB DDR 3 RAM (1600 MHz).
After putting the RAM in empty Sockets OS was not able to detect the RAM,  the issue seems to be due to limitation of 32 bit OS.
(http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778(v=vs.85).aspx#physical_memory_limits_windows_7)

Can i upgrade desktop to 64 bit OS using the same windows product key provided with machine. If yes how to get the OEM DVD for windows 7, 64 bit OS ?