Hello guys so i have this friends PC and i checked his specs i think he has got 2GB DDR3 RAM 667mhz or 800mhz ( dont really remember) so he is looking forward to upgrading it to 4GB so he has got another slot free and would like to add a 2gb ram. i checked online and there are rams with 1333mhz also ive heard both ram should be the same mhz if you want best performance. so please help me out. Also i would like to know whether the website crucial helps us to check what ram i could go on. so i would also like to know more about this