There is no disadvantage to buying RAM rated for a higher speed, except you will pay more for it. Remember, the "speed" of the RAM is the maximum speed the RAM is rated for
. There is no harm in "underclocking" the RAM, though the extra money you will pay will be wasted.
I don't want to make a statement saying that OCZ's suggestion is a load of bologna, because I'm sure they know that kind of thing way better than I do. However, I would take Intel's advice in this situation, especially seeing as you are currently having problems.
If I were you, I would buy some 1.5v 1066 DDR3 RAM, from a reputable company such as OCZ, Crucial, etc. Newegg ratings are helpful too.