2R means Dual Rank memory (as opposed to 1R, single rank)
x16 / x8 are just standards for how dense the memory is. Whatever you do, you should get memory that matches.
I personally would get one of these kits, which will be the higher performance type (the same as your original) Also, you know that they will work together, since they are already tested. http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007609%20600006176%20600000426%20600000397&IsNodeId=1&name=2GB%20%282%20x%201GB%29