The short answer is that the GTI 650 with 2 GB makes it better since the GDDR5 on the card has a better bandwidth. A 2 GB memory card is just as good right now as a 4 GB card in fact most games run fine with 1 GB of GDDR (see toms hardware tests). As for the comment about 4 GB cards not being available many manufacturers such as HP have computers with a GT 640 with 4 GB. A GT 640 with 4 GB seems to run most games at very high settings with no problem. If your not a serious gamer and don't need bragging rights the GT 640 with 4 GB will be fine,