What video card should i buy?

I just got a brand new computer. It works fine all except the fact that the performance rate for the game graphics is 3.5 which is pretty bad. I tried playing minecraft and it was lagging a lot. There was two video cards i was looking at, the Galaxy GeForce GT 520 2GB and the PNY Nvidia GeForce GT 520 1GB (Bestbuy.com) Which would be better? they are both PCI Express and GDDR3 but does the GB matter? So i am wondering which will boost my performance rating more? or will they do about the same? Thanks!!|||If you are planning to play games on 2 monitors simultaneously (dual monitors) then get the 2 GB but if not then the 1GB would be enough. Basically they have the same performance.