Both Intel and AMD have included the memory controller in the processor for some years now.
This means the default memory speed to be used is based on the CPU.
The motherboard should support a wider variety of speeds.
Faster memory will operate the slower speed supported by the CPU unless over clocking.
They also confuse things in the way these products are advertised.
Take DDR3-1600 for instance. This can also be called PC3-12800. It actually has a data rate of 1600MT/s, peak transfer rate of 12800MB/s and an I/O bus clock of 800MHz. It essentially uses the up and down of the signal (hence DDR or double data rate) and this is how they come up with "1600MHz" but this isn't really the clock rate.
Thanks....How do I know what ram speed the CPU supports?...I expect to get a 4770K CPU in my new build in april/may, so what ram should I pick