I've heard that Corsair ram run really well in ASUS mobos - have a choice between XMS 2 PC2-6400CL4 and CL5 for about the same price clocked at 800MHZ and a slightly more expensive XMS2-PC28500 clocked at 1066mhz, of course there's always the XMS2 PC6400 DHX-CL5 at 800MHZ. What would you recommend? My probable mobo runs at 1333mhz and is an ASUS P5K-SE.
CL is CAS latency, which is a delay.
CAS3 is better than CAS4, which is better than CAS5...
DDR2-800MHz memory will allow you an overclock, from 333MHz (FSB1333) to 400MHz (FSB1600).
Therefore you might as well buy PC2-6400, although you technically require only PC2-5400 @ 667DDR.
533MHz memory (@ 1066DDR) will give you memory bandwidth far beyond that of your CPU!
This will not add tremendous speed to your rig, but can generate impressive scores in memory bandwidth benchmarks. Some folks are into that... heheh.
But a nice PC2-6400 C4 will give you all the performance you desire,
it would only be a minimal difference between PC2-5400 CL4 675mhz and PC2-6400 CL4 800mhz?
Well, there is no difference @ 333MHz (667DDR).
The difference is that PC2-6400 is rated for 400MHz (800DDR).
The extra FSB headroom is useful even if you don't really 'need' it right now - so PC2-6400 would be a good investment. Running at 667MHz it should give you excellent timings @ 1.8v, and could run faster in future if req'd (ie. FSB1600).
So even though technically you require only PC2-5400, the 6400 is a good buy if you are a performance-overclock-gamer type guy (any two of the three will qualify you).
The main purpose of the PC2-6400 would not, however, be to run @ 400MHz with a 333MHz CPU (using a dopey 5:6 FSB:memory ratio, like it probably screams on your motherboard's box).
A nice 1:1 ratio with tight memory timings is all you need, and PC2-5400 C4 will do that.