mouad :
and can i use the NZXT LEXA S case and a segate baraccuda 500GB and the ASROCK Z87M PRO4 motherboard because that's what's available in my country
No you don't need 16gb of ram,8gb is more than enough and all of the above will work only the motherboard might not be so good for overclocking,but if that's what you can get go for it.
The motherboard also doesn't support sli so beware of that.If you go for the gtx card a 520->650watt power supply will do fine like the one in the set-up Waghzufel posted.
There simply are no other options?