On A64, I always aim for the highest clock speed first. Then manipulate the LDT, multi, and memory dividers to max out the memory (either lower speed/tight or higher speed/loose, determined after testing).
AXP - same as above, but leave the memory dividers at 1:1 and aim for the tightest timings.
P4 - no choice but to maximize the fsb to the cpus max, then select memory that suit your cpus max fsb either via 1:1 loose or 5:4 tight. That is for 865/875 dual channel setups obviously.
I will never sacrifice very much clockspeed for memory speed.