While in top type "M" .. that's capital M so "shift+M".
That will sort by memory usage. Find out what processes are consuming the most memory and then try to figure out why.
I'm also wondering why your stats are showing 77% cpu idle (which is not good) yet the top process doesn't seem to be using much cpu at all (it sorts by cpu usage by default).
I would be very interested in seeing your apache configuration .. since you only seem to have one apache process running.. or at least one near the top ..
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