From my experience, I think you should use a divider to find your processors max speed. Then go for a 1:1 ratio to get your ram up as high as possible (use loose timings to get a max). Next lower your timings on your ram.
I've found that higher cpu clocks help out more with multimedia than ram does. However, run a few tests, sandra lite, 3dmark05, and see which one gets you a better score. (a divider, or 1:1 ratio) of course this is assuming your processor gets to a higher oc than your ram can take!