|"also i want to upgrade my RAM"|
That should be your 1st move & it should be at least DDR3-1333 but DDR3-1600 would be better.
"The PSU is 400 W"
What make/model? If it's a low end cheapie, you might wanna consider upgrading it before installing a video card. Your CPU alone is rated at 125W. I assume the video upgrade is for gaming? GDDR5 memory is the way to go.
Have a look at this article: http://www.tomshardware.com/reviews...
message edited by riider