Run this simple test to verify that a gpu upgrade would be useful to you:
Run YOUR games, but lower your resolution and eye candy.
This makes the graphics card loaf a bit.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
What is the make/model of your psu?
It needs to be strong enough to support your gpu upgrade.
When upgrading a gpu, make it a significant jump or you may be disappointed.
Look for a card that is at least one tier higher on tom's gpu hierarchy chart:
We've run hundreds of GPU benchmarks on Nvidia, AMD, and Intel graphics cards and ranked them in our comprehensive hierarchy.
www.tomshardware.com
Since you now use nvidia, stick with nvidia as an upgrade.
Things will be simpler and at any price point performance differences between nvidia and amd are not great.