Hi Biohazard7
Welcome to the ROG forum.
You'll need at least 16GB of ram for the TitanX-P and it's probably not wise to spend money on upgrades for old hardware.
The 980x was a great cpu in its time but it's an old cpu now and will definitely hold you back. If you could swing a maximus viii board, 6700k and a fast 16GB DDR4 ram kit (3000MHz - 3200MHz) that would be the way to go. You should upgrade your system to get the full performance out of the new gpu's, that is if your budget allows.
With the 1080 and TitanX-P you could jump up to a 1440p monitor and have much better clarity with your games, I'm gaming at 1600P with the ROG Strix 1070. (60FPS)
You could even upgrade to a 980 Ti which would still destroy your 480's, my ROG matrix platinum 980 Ti is about 15 FPS faster than 680's in SLI.
So in short:
1) 16GB of ram with your system overclocked to the max is still going to hold back the new gpu's quite a bit.
2) No, a gtx 480 used as a PhysX card will slow performance.
3) Look at the 1440p monitors.