I just built a system in the same case. The 120mm fan in the rear does an
excellent job of moving air. I did add the 80mm in the front for intake. I
did not put in the side fan, as 80mm fans tend to add a lot to the noise
level. I did extend the air tunnel so it was closer to the CPU fan,
hopefully giving it access to uncirculated, cooler air. If you use the side
fan, it's 80mm.
I think the only reason it would be necessary to use the side fan is if you
are setting the system in a confined space, or if you have a video card that
would benefit from extra cooling, in which case you would want to remove the
air tunnel so the air is not funneled just to the CPU.
I would consider adding a filter if you were going to use the side fan for
intake, otherwise you'll be blowing dust and lint directly onto the CPU
fan/heatsink.
I've never been more impressed with an inexpensive case- great product for
the money.
Good Luck,
Fitz