AhmedSaeed :
i checked the psu n it says 240w so i think gt 750 ti is out of question. recommend me any other card so that i may b able to play assassin creed unity n witcher 3 (at average performance or medium)
There probably isn't one that will run on that PSU.
http://www.systemrequirementslab.com/cyri/requirements/assassin-creed-uniy/12370/?p=r
http://www.systemrequirementslab.com/cyri?itemId=12446
And I don't know what CPU you have.
That PC was never intended for playing 'A' games on. You might be able to run a GT 720, but I doubt it would play either of those games well at 1080p even with minimum settings.