mobo memory compatibility

Solution
I would just whack in the cheapest RAM that I could find that matches my requirements, looks wise etc... also look for RAM with decent timings at atleast 1600MHz @1.5v and you'll be set.
so everything will be fine itll just be capped at 1866mhz?
or would it just be better buying ram that does 1866?

and my processor is a am3 phenom II which i used to upgrade my aging mobo 2 years back and i plan on upgrading to FX
 
ok thanks!