do the cards have to have the same mem and clock speeds... cuz im gunna SLI eventually but i dont have the money right now.... and when i do i donjt wanna have to go through all this if newegg discontinues my card... im getting a MSI 7900GTX with 650/1600 clock speeds so if i were to get another would it have to be the same cuz i know some companies are +-25 mhz on some things