Hi Smoogle,
Welcome to Insights!
Based what I read in the net the H1Z1 game will run if you have this PC requirements based on Planetside 2 game;

For Intel users with Nvidia GPU:

• Core i5-661 3.33 GHz
• NVidia GeForce GTX 560

For AMD users

• Phenom II X4 910
• Radeon HD 6870

These are the recommended requirements but what about the minimum requirements?

For Intel users with Nvidia GPU:

• Core 2 Duo E6850 3.0 GHz
• Nvidia GeForce 8600 GTS 512MB

For AMD users

• Phenom II X2 555 Black Edition
• Radeon HD 4850

Therefore a R9 280 will run fine based on your system specifications.
In regards to the other two ports you mentioned, those are Mini DisplayPort outputs. The Displayport output has the same benefits as HDMI as it can transfer video and sound but has a higher bandwidth as you can run resolutions up to 4096×2160 at 60Hz. 
If you plan to add a third display that is not DisplayPort capable, then you have to use an active adapter on those ports to make the proper conversion to whatever connector you want to use. If you want to add a fourth display you need to do the same.
The R9 280 is capable to run up to 6 displays but you need to use a MST Hub.

If you have further questions please post here.

Regards