use the fastest mem speed where you get NO artifacts at all (artifacts are wierd mishapen, discolored polygons, or incorrectly colored pixels). if you jump up in 10 mhz increments, in games eventually you'll see them. if you og way too high, you start to see htme even in 2d....
use the fastest core speed that doesn't lock your coimputer and doesn't make the image look funny.
I personally back off 5 mhz from the fastest speeds that appear correct, just for safety, and since there is no visible performance loss.