those dividers are there so that FSB and RAM speeds can be different, because the RAM might not be able to handle the FSB speeds otherwise, or because the RAM can go further than the FSB. 1:1 (for example: 333FSB / 333RAM -> 1333QDR / 667DDR) is generally the best, but 1:2 (333FSB / 667RAM -> 1333QDR / 1333 DDR) is perfect. nowadays, 2:1 is used for DDR1, 1:1 for DDR2, and 1:2 could be done with DDR3.
other dividers, like 2:3 (400FSB / 600RAM -> 1600QDR / 1200DDR) are sometimes used to push the RAM to its limits, while keeping FSB within limits and OC with CPU multiplier instead. in the mentioned example, this provides better performance, as the MHz increase of the RAM outweighs the advantages of 1:1. (if the NB and RAM can exchange data at the same speeds, there arent any wait-periods. if the NB and RAM run at different speeds, the fastest of the 2 has to wait for the other to accept the data, due to this 1:1 outperforms 4:5 or 5:6, even though the latter 2 have higher memory speeds)
so in your case, as you have DD2-800, 2:3 would give you memory speeds high enough to outweigh the 1:1, but real life performance (games) doesnt notice the difference, so i advice 1:1. this keeps your RAM a little safer.