I have a cable and i dont know if it comptability with a dual molex

Solution
So a standard ATX form factor PSU will be just fine.
http://uk.pcpartpicker.com/product/NrDzK8/evga-power-supply-220gs0550v3
http://uk.pcpartpicker.com/product/3hkwrH/corsair-power-supply-cp9020102na
http://uk.pcpartpicker.com/product/TgW9TW/seasonic-power-supply-m12ii520bronze

Also just so you know that CPU is weak(poor single thread performance) and will hold back a GTX 1070 from performing to it's full potential. For your next upgrade you are going to want a new CPU, motherboard and ram(newer cpus use DDR4).
ATX Switching Power Supply Model: ATX -500B This is my PSU 500W is it enough for the card doesn't it? I dont really wanna buy a new psu i dont even know what goes where in, im a newbie.
 
That psu is utter garbage. It has an extremely weak 12v rail 22a which is inadequate for a system with a gtx 1070 using under that kind of load is a danger to the system add molex adapters to that and it becomes a fire hazard.Trust me this is not a lesson you want to learn the hardway.

As for how to replace a psu hit up youtube there are videos out the wazoo for system building and parts replacement. That will give you something to do while you wait for the psu to arrive.
 
Its a pre-build pc

Motherboard: GA-F2A68HM-HD2
Memory: 16gb
Hard drive 1TB
PSU ATX SWITCHING POWER SUPPLY MODEL: ATX-500B
Case: Fierce Sandstorm Red
CPU: AMD A8-7650K Radeon R7, 10 Compute Cores 4C+6G

Im in the UK
 
So a standard ATX form factor PSU will be just fine.
http://uk.pcpartpicker.com/product/NrDzK8/evga-power-supply-220gs0550v3
http://uk.pcpartpicker.com/product/3hkwrH/corsair-power-supply-cp9020102na
http://uk.pcpartpicker.com/product/TgW9TW/seasonic-power-supply-m12ii520bronze

Also just so you know that CPU is weak(poor single thread performance) and will hold back a GTX 1070 from performing to it's full potential. For your next upgrade you are going to want a new CPU, motherboard and ram(newer cpus use DDR4).
 
Solution
The performance loss will depend on the game a game like bf1 the 1070 might be at 50% usage(just a guess) a d it also depends on the resolution the lower thevresolution the bigger the bottleneck. Current gen and upcoming cpus and motherboards use DDR4(there are some 1151 boards that dan use ddr3 but it's not reccomended due to the higher voltage they use).