how many fans should you have in your case? is the normal psu fan and 1 120mm fan at the back fine? or should you have 2 120mm fans? what would be best if you have a quad core and a video card? and how much power do fans use? (none LED)
Are you overclocking
or intending to at all?
does the pc live in a cubbyhole or under a desk?
whats the airflow in the room like?
does the gfx card have its own fan?
and possibly more factors I cant recall atm
You should be fine in most scenario's with one 120 intake (front or side usually)
and the rear exhaust,
let us know the answers to my questions and well have a better idea of your minimum needs,
fans use a tiny amount of power btw, unless you have a ton of them, even then its no major amount I had 8 fans off one molex line (7x80mm,1x120mm, all led's) then a lightbar set up and my hard drive until recently
I'd reckon putting another 120 in wont hurt it any, make sure the exhaust fans have clearance to get hot air away from the pc, and intake (if you have one) has a clear pull of cool air to bring in.
hope this helps,