ram 2gb 2R x 16 PC2 .... what does it mean ?

have a macbook white 13 " early 2009. I want to upgrade ram from 2 gb to 4 gb

I have found a samsung set which reads: 2 gb 2R x 8 PC2 - 5300S - 555 - 12 E3

the one that i have on at the moment reads :1 gb 2R x 16 PC2 - 5300S - 555 - 12

They are both 200 pin DDR2 667 MHz

Now the only difference is in the 2Rx16 or 2Rx8 and i don't know what this stands for and therefore i don't know how important it is for the performance of the macbook, can anyone help this poor ignorant ... pleeeease. Thanks everyone.