It depends on what you are doing with your PC. If you are referring to gaming, your GPU's would be the first place I would look. A good GPU upgrade would give you better performance across the board in all games. A CPU upgrade will only give you better performance in games that take advantage of the newer architecture.
I'd recommend a GPU upgrade and if you want to take a hand at overclocking your CPU, that would give you the best upgrade.
If you are not referring to gaming, then we have to look at what apps you are questioning and pick them apart. I will not go into it because we could be here all day.
The Phenom x4 965 is a decent cpu, I had one in my old build. BEtter than Bulldozer. Anyway, I would upgrade to a 7870, depending on your budget. If you're looking to go more expensive try 2x 7870s, or a single 7950/70