For most this will be obvious but still for some it might be of some interest.

Here's a little math about memtimings:

To know what's fastest setting, the first thing we have to know is the cycletime as most memorysettings are just the number of cycle times, for example, CAS3 means 3 cycle times.

Now to get that cycle time we do a littlle math: t (cycletime) =1/f (frequency).

So, if frequency varies, cycle time does as well.

Here goes:

400MHz (DDR800) 1/0.4GHz= 2.5ns (nanosecond)
500MHz (DDR1000) 1/0.5GHz= 2ns
600MHz (DDR1200) 1/0.6GHz=1.66ns

Now running DDR800 with for example cas4, we have 4x2.5ns=10 ns latency, running cas5 at that same speeds gives 12.5ns latency and so on, needless to say, the more latency the slower the run will be.

Now it becomes obvious that running 5-5-5 at 600MHz is almost as fast as 500MHz 4-4-4 and 500 5-5-5 is as fast as 400 4-4-4.

Hope this helps a little.