My PC Specs;

8 GB RAM Kingston 1600MHz DDR3.
i7 3770.
Cooler Master Mwe 550W PSU
ASUS-P8Z77VLX motherboard.

I was using this system with GT 640 and a bad PSU(which is 350W).

Then I replaced GPU first, but I couldn't feel almost no difference. I thought maybe PSU was the cause of this problem. Then I replaced that too. But nothing changed. I just wanted to play recent games on low, even BF3 with medium settings 60+ FPS. But no BF3 very low settings gave me 40-80 on Caspian Border(Which is a big map). HitmanTM with 40 FPS on medium. BF5 online with 20-30FPS. I looked for the benchmark with my GPU+CPU combination, they can easily play APEX Legends, BF5 and much more games like these. Please help me, in my country these are not easy and cheap things to buy. Please try to help me, I will appreciate it.

(My motherboards BIOS version is not the latest one, not the latest but ok. I will try to update it tomorrow, but I don't think it's going to help.)

I haven't seen that this GPU had more than 100W TDP ingame.

I will try to answer any question you will ask.