okay fellas....here's the conclusion that i have reached:
if you want raw speed in your ram:
go for th HZ's. i have managed to get them to run at 1100+mhz with 5-5-5-15-2t timings.
they will not run over 950ishmhz with cas 4, even with 2T command rate.
they also run great at 800mhz with 4-3-3-8 2T.
they will do 4-4-4-12 1T @ 800mhz, which is pretty good (most ram is made to run at 2T command)
all these settings at 2.2V
this (the 1100mhz) will give you a better time in most benchmarks, definitely a better time in super pi.
so...if you are getting a c2d rig, and your concern is absolute speed in your ram, go gskill.
if you want low latency, with good timings:
go with xms2 c4.
i can make them run at 950+mhz with 4-3-3-5 1T (and thats f**kin fast boy-o) with 2.4v
i can get about 1000mhz with 4-4-4-12 2T with 2.3V
with 5-5-5-15 2T i can go to about 1050mhz
so...if you are buying (or upgrading) an amd system, your latency will be more important than your raw speed...
so get xms2c4.
thats my conclusion from both sets of ram i own. take from it what you will, my friends