With the current cpu, doing some research you can go for = click
It's an XFX HD577AZNFC Radeon HD 5770
(A little old).
Review = click
But if you want to upgrade your PSU, like go for Antec or XFX (like me) You can for NVIDIA.
Im searching good prices over the net.
The problem it's i have to the conversion to UK to dollars.