ddr -II-667 = Value Ram = High Latency = lower oc'ing potential
XMs2-800 = daddy of rams = high end ram = lower latency = higher oc'ing potential
that being said:
i am running el-cheapo kingstons KVR-667 and as long as i keep my fsb such that i dont go beyond mem specs i am okay. infact i know of people who have run this ram at 800mhz effective no issues. ofcourse the latency tends to be higher
as for the latter. the, there are 2 listed on corsiar's website if i am not mistaken CL5 and CL4. CL4 is obviously faster as it's CAS is 4, but i am sure the other would probably also run at 4.
one thing to keep in mind is that high end ram tends to require more juice than the std ddr-2 affair of 1.8v. many intel made mobos based on the 965 chipset didnt have the requisite info in the bios that was released with the initial batch of the boards. support for higher juice was only added later. make sure ur mobo can support more juice to the ram.
if i were u i would get more of the el-cheapo variety unless u must really push ur fsb such that it goes well beyond the 667.
e.g. the fsb for all core2s on 800mhz bus -200 quad pumped. the ram at default is then running at 400 effective.
most core2's are on a 1066 bus, thus the base fsb is 266 quad pumped. ram is thus at 533 effective
the newest core2's are 1333 bus... u get the picture
so if u want to oc a 1066 cpu u're okay till u get to 333 fsb. which can take u places depending on the clock multiplier on ur cpu.
e.g. mine is 9; 9*266 = 2.4ghz base.
it is oc'd to 3.0ghz using 9*333= 3.0 ghz, mem running 1:1 at 667