Looking to upgrade my graphics card...

Basically that's it. My NVIDIA 8500GT isn't cutting it these days with some of the games I've purchased and PoE runs okay at a lower resolution without shadows, but I want more!

I'm not very tech savvy in this area so I need some help from the community.

This is my computer for reference.

The only thing I do know is that I'm limited in my options since my factory psu is only 300w. I'm also a little constrained space-wise so anything bigger that what I have I fear won't fit. If you guys could fill me in on what I need or make suggestions that would be awesome.