Does adding more RAM give me any performance benefits? If only 3.33GB of my 8GB DDR3 Ram is used, does this mean if I upgrade to 16, I'm doing something useless because it won't be used anyways?
You're not always using 3.33GB of RAM, some days you might use more. However, if you don't get close to using up a lot of your RAM that often you probably don't need to upgrade.
With the limited information given above, I'd say you wouldn't notice an increase in performance upgrading to 16GB of RAM.
Does the fact that 8 diagrams are shown mean I have 8 cores? if not why 8 rectangles?
It means you have 8 logical processors. Observe, the Intel i7: it has 4 cores that support hyperthreading. If HT is enabled, a Task Manager running on an i7 should show 8 graphs by default, one for each logical processor.
Trivia: You can coalesce all 8 graphs into one, if you want to see total CPU usage.