Looks like a nice well breathing case! I suppose you have one fan in the rear like the picture shows? Add one 120mm on the top and another in the front. Both upper fans should blow out, the front one in. Keep the front grille dust free, ensure there's enough space around the case - the so called computer desks with a closed door cabinet for the case are the worst thing you can think of! I also suppose that the air intake for the power supply is in the bottom, in which case you might like to add height to the feet. Something like door bumpers will do the trick, leaving 1" or more space beneath the case. If your video card still runs hot, adding one fan on the side blowing in will help.
As for noise level, decibels don't multiply by the amount of fans. If you start with, say, one 20 dB fan, two will make about 22 dB and three about 24 dB. For more exact calculations, decibels go in a logarithmic scale. Below 20 dB is hard to tell from the overall hum.
