A Ram Question

Hello, I would like to add/upgrade my ram. I have an Alienware (yes I know, slap myself) Aurora R2, i5-655k, HD5870, 6GB (4 banks, 2 of 2) DDR3 PC3-10600 1333 MHZ CasLat 9. The physical config(?) is Dimm0 & 2 contain; 2GB SamSung p/n M378B5673FH0-CH9, Dimm1 & 3 contain; 1GB Kingston p/n KTW149-ELD. I assume that the banks (4) are physical slots and my max memory potential would be 16GB? I am unsure what "2 of 2" means. Does this mean each pair of bank /slots can be 4GB modules (matched set)?. Any clarifications would be hepful. Thank you, Dennis