Old PC, new GPU: Will it work?

You helped me put my current rig together quite some time ago, so I figured it was time to reach out again. 🙂

Is it worth upgrading just the GPU on this build?
Say, an EVGA GTX 760 gaming 4 GB GDDR5 or something similar.

Current specs: Quad Core Q9550
Gigabyte EP45 - UD3R motherboard (LGA 775)
4GB DDR2 RAM

For context,
I'm currently running an Nvidia GTX 275.

I'm not looking for miracles, obviously, but if it's feasible...

I honestly don't think the gtx 760 is worth the money here, and it’s probably overkill for that CPU anyway. If I were you, I'd maybe just aim for an R7 265 at most.

If you manage to overclock your CPU to around 3.4GHz, you might be able to squeeze out an R9 270 at most. I guess that processor is just a bit weaker than an Athlon II X4 750K or 760K when it's running at stock speeds.