You may have to change the BIOS to get the faster figure, along with upping the ram voltage. When it comes to latency values that are published by companies, I've come to the conclusion (which may be wrong) that the advertised latencies are what the ram is capable of, but not necessarily what a person will get. Its kind of like the EPA stickers on cars that estimate the gas mileage; you might get that high a mileage, but probably not.
Oh yes, faster advertised latencies are still better than slower advertised latencies, same as in cars with better EPA gas mileage.
1.8V is the specified voltage for DDR2 So in order to meet those specs, they will come with relaxed timings of CL-5 that will work at 1.8V...think of those as universal defaults for DDR2. These are JEDEC standards which are set so that everything will be compatible with one another. As part of the specs, your BIOS defaults to 1.8V as well so if you were to put in a set of RAM modules with default CL 4 settings that requires 2.2V, you would not be able to post because you would be undervolting your RAM.
CL-4 advertised RAM will default to CL-5 to meet the voltage specs in order to ensure that your system will post. After that, you will have to go into the BIOS and alter the latencies and voltages just as Sailor has said.