You have to get two of same exact gpu's, so the same exact cards pretty much. There are different clock speeds on some cards that are the same name. I don't understand that last question... 2 cards of the same 256mb? You mean, two 128mb cards to form 256mb? Please enlighten me...
ya sorry i worded that like crap. I mean like to cards that are 256mb or two cards that are 128mb. So what ur saying is i should get the exact same card. Could i only get one card on a mobo that can hold 2?