1Rx8 has the memory on one side only. 2Rx8 is a two-sided memory stick.
That's not true, Blue. I have, right in front of me, two sticks of Samsung double-sided DDR3 (one 1G and one 2G), and they are both labeled "1Rx8 PC3" -- and they are both double-sided.
"1R" means "single rank," and "2R" means "dual rank" organization.