I want to upgrade my PC..

.. But I need help from you guys!
I'm a noob at this stuff. (Always been a console gamer)
What I currently have in my pc;
ATI Radeon HD 5670
Quadcore AMD Phenom II X4 925 Processor
4GB DDR3 RAM

What I'd like to have;
A better video card
4 more gigs of ram.

But I don't know what to buy first and where to buy it from. HELP.