I'm using the power numbers from w1zzard's review of the MSI GTX 1080 Gaming X Plus 11 Gbps 8 GB
Okay. Officially, your power supply does not support that GPU.
Officially.
A 520 watt PSU
should be big enough for what you have.
You may want to read JG's review and explanation of the HX520's "rails":
Corsair HX520W 520W
A PSU that can put out 480 watts on the 12 volt rail(s) should have no problem with a ~235 watt GPU. I'm no PSU expert, but I think it can handle it, just get a
6 to 8 pin adapter. Here's my reasoning. That PSU has the following rails rated at 18 amps each:
12V2: PCI-e 1 and first two peripheral connectors
12V3: PCI-e 2 and last three peripheral connectors
ATX rated PCI-e power from connectors:
PCI-e slot: 75 watts
6 pin PCI-e connector: 75 watts
8 pin PCI-e connector: 150 watts.
Therefore, your PSU is set to provide a GPU 3 x 75 watts=225 watts. But those "rails" are rated at 18 amps, which is 216 watts EACH. Running a few watts over the rated capacity for a 6 pin connector is not a big deal, IMHO. AMD went over rated capacity for the R9 295.
You don't need a 700 watts PSU.
Here's the specs for the Seasonic FOCUS PLUS GOLD SSR-550FX
Notice that there is 540 watts available on the 12 volt rail. You only need to go bigger than 550 watts if you think there's (something like) a GTX 1080 Ti in your future.
Please don't buy CM PSU's. They make some real JUNK PSU's so I think they should be punished for that.