Three: MSI GTX650 Ti Boost 2GB DDR5 with out additional RAM.
3, no question asked... it won't even be close. the 650ti BOOST is almost 3 times the gpu the 640 is. and you don't need 8gb of ram, so i wouldn't worry about your ram today. get the best gpu you can and enjoy the ride.
btw: the reason why your sister can game better is resolution size. she's playing on a tiny monitor at very low resolutions.
What I normally do is stop Explorer, Themes and AVG. But I have to turn off shadows and fullscreen. just to play at a decent frame rate.
But thanks for the advice. I was originally planing on Corsair Vengeance Pro but I realize the RAM is not going to help me much. I will then go for the GTX650.