Before memory controllers were integrated into the CPU, they actually operated at higher clock speeds. The CPU integrated ones are still faster overall, due to not having a delay between memory controller and CPU, but for those discrete memory controllers (north bridge of chipset), you'd want the higher RAM. It's somewhat ironic that you've tagged your question fsb and then mentioned the Core-i line and AMD APUs, none of which have a Front Side Bus.
The "supported" RAM speed of those CPUs is the officially supported speed. Overclocking is quite common.
Even when the RAM chip is used at a lower frequency, the higher-spec RAM chips will have fewer wait states between e.g. address stable until first data word read back. The clock speed controls the burst throughput within a RAM block. There are many many other timing factors affecting system performance than burst throughput.