What would be the best graphics card for PC gaming without having to upgrade my PSU. I dont see any extra power cables running out of the PSU to connect to a video card either I'm running a 24inch monitor if that helps any.
For starters, you have a great CPU.
A decent GPU should make that PC a somewhat of a mid-range gaming PC.
What resolution do you play at (I assume 1900x1080)?
The best card available that can be run with a 300W PSU is the HD 6670: