What would cause all the memory to be used up, but not display with a TOP command?

Like the title says, i'm running a dedicated server for some personal websites, which receive quite a bit of traffic.
The server load is quite high at the moment, and has been getting this way for the past month or so.
So the first thing I've done is a TOP command, to see whats going on.
I've attached it to this thread.
How come it says all my memory is being used up? But the processes says it's not using anything?
I'm assuming this is whats maxing my CPU, creating swapfiles on the hard drive?
Do I need more memory? Or is there something else that doesn't meet the eye?

 

 

 

 

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