I have a set of $450 Dual Channel DDR sticks in one machine. They are said to overclock several hundred MHz. I'll be damned if I can get 10 out of those sticks, regardless of voltage and I know my shit.
I have seen another guy with cheap memory take is up 100MHz. So there are no guarantees.
I've also overclocked RAM that would let me into Windows, but once I started working the computer would crash.
Never believe anyone who tells you you are guaranteed an overclock in any rig. Always buy the best and fastest RAM you can afford, or there's a good chance you could be kicking yourself in the ass later.