When you click on the "Free RAM" button SpeedUpMyPC attempts to recover at least 10-20% of the currently used RAM but leaves the rest of the currently used resources to the applications currently using them. The "Free RAM" function is enough to free considerable amounts of RAM in most cases and you can execute this operation any number of times in sequence to free up more and more resources.
Basically, it depends on how many or what kind of applications (ie. memory intensive ones)you are running.
Also, when using the automatic feature, make sure that the value set for x% is a reasonable one, that is, 75% or higher. Please, note that this percentage is a target that the "Free RAM" functions tries to maintains not at all costs!
