You can use both. The bottleneck is the CPU, and other devices will clock themselves down accordingly.
I would still go for the higher one, tho: While it won't give you any benefit at all now, it would allow you to use the same RAM if you were to upgrade to a CPU with a faster clock rate.
The rule of thumb is that RAM will run as fast as it can and is allowed to, whichever is lowest. This is OK provided that the RAM can run at least as fast as the CPU.