As always, wikipedia shouldn't be used as a reference source, but the basic information is accurate. http://en.wikipedia.org/wiki/CAS_latency
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.