I would say 8500 is absolutely not worth it.
On the other hand, let's face it, no high-end hardware is value for money.
With people saying "if you want to push your CPU, you need 8500+ RAM", I can only see this being an argument for those with 45nm CPUs with low (<9x) multipliers. As soon as you hit a 10x multi, you're pretty much sorted. Just run the RAM 1:1 if you have 6400. Running 8500+ isn't going to show much difference as long as you run a low tRD on both systems.
Lowering the tRD gives the most benefit to memory latency and bandwidth and comes 'free' with your motherboard (assuming it has the ability to change it - most overclocking boards do).
e.g. On Asus P5Q, I believe it's called "AI Transaction Booster". Lower = better.