Your system will perform a little faster with CAS 6 or CAS 7. Whether or not that's noticeable in day-to-day usage is another matter. However, CAS 7 isn't that much more expensive than CAS 9 most of the time, so you might as well at least get 7, especially if you're already dropping a bunch on an i7-920 (why not a 930?).
You might not notice lower CAS during short periods of gaming, but if you're doing significant number-crunching or editing, or you want that extra edge vs. your opponents in an FPS game, I would expect it to be noticeable in the long run.
Srry I don't remember where I read it but I'm pretty sure it said this: "the recommended voltage for ram for the i7 is 1.5v(+/- 5%) otherwise it could damage the cpu." And If I would overclock I would buy an NH-U12P SE2 or a similar one i think that should be ok am I right?