The memory your apache is using depends from php memory usage, from number of loaded apache and php modules and the number of connections.
You always want to check for the worst scenario:
MEMOF( max php mem,max loaded modules) x max apache connections
and you always want to avoid going into the swap so the result should be less than your physical memory amount.
You also have mysql server running so you have to consider it max memory usage as well.
You can do many configuration optimization to reduce the memory usage while keeping your site fast and stable ( do a search in serverfault - here you can find useful discussions).