I think you are looking at many levels up on the GPU. Since MODs can take a big bite on performance I don't know at what GPU you need to stop.
At high or ultra on the 1080p screen I think you're looking at the Nvidia 1070.
Look I know Jack about pc parts. Simple as that. So if you start talking about speeds and other parts and complicated stuff I will have no idea. So just tell me if these parts will allow me to play gta v with mods @ 60fps with good graphics as I love the graphics on a game. I would love to run it at high or ultra but tell me what it could do.
AMD FX-4300 3.8GHz Quad Core Processor
ASUS M5A78L-M LX Plus [VGA] {2 DDR3 Slots}
8GB (2x4GB) DDR3 1600MHz Memory
1TB Hard Drive
Nvidia Geforce GTX 950 2GB (Min. 400 Watt Power Supply)
I just wan't to play the game and i'm not looking at going faster and over 60fps so If this will play GTA V with mods installed and at 60fps with good graphics i'm happy with it. If not list a part which I need to upgrade and i'll see to it.
Thanks.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic