Problem is, it tends to be the cheaper, lower end boards that support both, like ECS with there SIS chipped moby's.
Nothing wrong with this, depends what you are after, but they tend not to be the fastest and most stable or best supported of boards.
It is a good solution if money for a complete DDR upgrade is tight and the SIS single chip is not a bad performer, if not right up their with Via.
I know I have contradicted myself a little here but I am just making you aware of the downside as well as the ups. Martin Just trying to help, sometimes falling short, I am only human after all.