It's somewhat pot luck, but in the majority of cases the specs that are described in your MB manual are accurate.
These specs describe RAM modules that are _guaranteed_ to work on your mobo. In most cases, there will be faster RAM modules existing that will work anyway -- but again, in most cases, faster RAM Module X is unlikely to work on a specific mobo Y.
In the 1990s it was not unusual to be able to fit faster RAM into any random mobo, but in the 2000s it would be unlikely to work -- RAM specifications are more stable and trustworthy than they used to be.
Besides, there is no guarantee at all of a stable system if you decide to circumvent any of these design specifications