But remember, if you get an 800Mhz cas4 , it is not a JEDEC standard so you might have to change it to a cas 4 in the bios, as most companies program their modules to the conservative JEDEC standards (which the highest standard is 800Mhz, cas 5).
Since 800MHz cas 5 is a JEDEC standard, therefore most of the time, it will be programed into the module.
So, if you dont want to go into the bios at all (no O/Cing at all), than 800Mhz cas 5 is for you, even tho cas 4 runs a lot faster.