A gaming computer for a first time builder/gamer

This is my parts list I Don't really want to change anything unless its that god awful my budget is what i have my final build at. I need the operating system and my only concern is the power supply i read some reviews and am concered a little bit.

http://pcpartpicker.com/p/QOuz

One other thing that i want to ask I see people buy all there parts from one place is that the best option or is buying them from different places okay?

 

I'd swap out the NVIDIA card for a AMD option of the same price, the 7950. Otherwise, great choice :)