Need to update my Alienware X51 R2

Hi, I have bought an AlienWare X51 R2 on ebay one year ago. And this is its detail:

I5-4430 CPU
8G ram
GTX 645 video card

Problem is, I want to update my Desktop to:
16G ram
GTX 780

Is that possible and how to do it ? Do I have to change my adapter (power source) as well ? Does Dell have any support team to help me (I send my Destop to them and they update it for me) ?