Your program is already using the on board RAM:

int a = 5; // This is RAM. Where did you think it goes?

DDR3 RAMs require (typically) 1633 MHz clocks, and store gigabytes of data, not something you use with Arduino, which has a 16 MHz CPU.

If you need EXTRA (emphasis on extra) RAM, my first advice would be to refactor your project, and reduce processing on Arduino.

However, if must have more RAM, get SRAM. In most cases, DDR3 trade high maintenance for cost. Have a look here.

Your program is already using the on board RAM:

int a = 5; // This is RAM. Where did you think it goes? (This might go to a CPU Reg)
int a[5000]; // This is RAM

DDR3 RAMs require (typically) 1633 MHz clocks, and store gigabytes of data, not something you use with Arduino, which has a 16 MHz CPU.

If you need EXTRA (emphasis on extra) RAM, my first advice would be to refactor your project, and reduce processing on Arduino.

However, if must have more RAM, get SRAM. In most cases, DDR3 trade high maintenance for cost. Have a look here.