It depends on your CPU, MB, and RAM settings, the programs you run, etc. You just have to experiment. At least on C2Ds, going one "notch" higher than synch (eg DDR2-667 over DDR2-533) may hurt more than help. However, going another notch higher (eg DDR2-800) should help. Lower latencies help at any speed, since that lessens the re-synching wait.