Even if you overclock the 955 BE to 4.0 GHz, it will still be the weak link. It's a good budget gaming CPU, but if you are playing modern titles that can use 6-8 cpu cores, the FX 8320 will give a noticeable improvement and support the 780 better.
Just make sure your board can support it. To get the most out of an 8-core CPU you want good cooling and a mild overclock. This means you need something like a Gigabyte GA-970A-UD3P motherboard with 8+2 phase power and LLC control in the BIOS.
You only need a 990FX board is you plan to go dual GPUs for Crossfire / SLI.
https://www.alternate.be/ASRock/970-Pro3-R2-0/html/product/1075727?tk=7&lk=6138
Not all AM3 boards can handle an FX 8 core. Double check before you buy it.