What are you using the system for?
If just gaming, more than 4gb isn't needed, unless your on a triple channel intel board, in which case 6 is your number,
If its a working pc, editing,heavy applications etc then go for the bigger amount and tweak the timings yourself,
*edit, 7 is faster than 9 btw, lower numbers are faster
I agree about not using any 3X__GB Kit on Sandy Bridge's Dual Channel. Most Photoshop projects even with a couple dozen layers and at high resolutions do fine with 8GB. If you're doing 'Production' then IMO it should be done on a WS MOBO + Xeon + ECC.
The item that will help the second most, beyond 8GB or 16GB of RAM, is the use of a fast 'scratch' SSD or use 16GB RAM plus a 'RAM Drive.'