Question Which GPU should I take for the most FPS

[Faronoz]

CPU: Intel Core i5 2500 @ 3.30GHz
RAM: 8.00GB Dual-Channel DDR3 @ 665MHz (9-9-9-24)
GPU: 1024 MB ATI AMD Radeon HD 6800 series (Sapphire / PCPartner)
SSD: SAMSUNG SSD 830 series of 119 GB (SSD)
Hard drive: 931 GB Western Digital WDC WD10EZEX-08WN4A0 (SATA)
Motherboard: MSI P67A-GD65 (MS-7681) (SOCKET 0)

These are the specs of my PC, and I get very low FPS with most games. For example at Fortnite I get 10-30 FPS with all settings on low, so just bad.

Now I can say that it is 99% because this bad GPU am i right?. So I have to buy a new video card right? If you have other suggestion for me to get more FPS/ better performance please let me also know that.

My budget is € 500 and I am also just willing to purchase a used GPU.
Now I searched the internet a bit and I am interested in the NVIDIA Gtx 1080, but I found very little about it, what I only found was the GPU’s with the brand Gigabyte, MSI. Then let me ask my questions: Which GPU should I purchase? Which brand is the best? What are the differences between the different brands of 1080?

 

[AngelTech]

AIB's is more preference than performance and if you have the money for a 1080 go for it! if you are squeezing too much money you could get a GTX 1070

 

[Faronoz]

AIB's is more preference than performance and if you have the money for a 1080 go for it! if you are squeezing too much money you could get a GTX 1070

-Oke, I am dont know much so what is AIB's
-So it will help me getting better perfomance if I put a better GPU right, some people said my system with I5 would not be able to benefit from a 1080 for example
-And i can not find a Nvidia 1080 but only Gigabyte or Msi what is the difference betweens all those 1080 with different brands and which one is the best

 

[AngelTech]

-Oke, I am dont know much so what is AIB's
-So it will help me getting better perfomance if I put a better GPU right, some people said my system with I5 would not be able to benefit from a 1080 for example
-And i can not find a Nvidia 1080 but only Gigabyte or Msi what is the difference betweens all those 1080 with different brands and which one is the best

AIB = Brands and like I said both of them will do just fine and I mean I like to research when answering questions to give the best answer possible and many people have had the same CPU and been fine with a 1080 you may experience CPU bottleneck in some games but it shouldn't be TOO bad maybe a few frames lost or a little more hard to say (if you don't know a CPU bottleneck means the CPU is holding back your GPU)