It's DDR3 1333MHz you need but your current 4GB Ram should be plenty. Aim for a Nvidia GTX560ti graphics card or better. PSU should be about 600-700W minimum. Buy a cheap one at your own risk. It's better to go with a well known brand such as OCZ or Corsair etc.
Microsoft Lumia 950 vs Lumia 950 XL: What's the difference between Lumia 950 & 950 XL?