I know this can't have a simple answer. A subjective answer would do fine too. The question is, for a gaming system, how much of a difference does it make to have faster ram? Is it always worth the extra cash? Is it negligible? Is it only noticeable to a few? Would it make sense to sacrifice speed in favor of capacity (slower 8GB Vs faster 4GB)?
If money is of no concern then go for the very best available, however for us mere mortals the trick is finding the best balance between price & performance. Memory seems to either get completely neglected & people end up with the largest capacity kit (often with high latency) they can afford once all the other parts have been bought, or huge capacity kits with insane speeds end up being thrown into builds where they will just be wasted. Higher speeds / lower latencies will provide slight fps gains in some games, but the extra money spent on that higher speed memory could have been better spent on a more powerful gpu, providing greater fps increases for your $$$'s. don't get me wrong, i'm not against purchasing a faster kit if the price is right, just consider what that kit will do for performance & how you could boost performance elsewhere by instead going for a more reasonable kit with decent timings.