Ideally you'll need both a fan at the front sucking in and one on the rear exhausting although a lot depends on the case. For a passive card I found that the addition of a fan on the side panel blowing directly onto it had the most effect.
I take it this is your desktop. If so the usual configuration is one fan on the cpu. one fan on the power supply unit and, often, one extract fan on the back of the case
Some computers do not have the last fan and if you want to improve the cooling that is the one to install. You'll usually see a circular pattern on the back wall with 4 screw holes round the edge to attach the fan inside the case. You'll often find a standard 80mm fan will fit and usually costs around a fivIf all 3 fans are working you should get a stream of warm air out of the back of the psu and the case fan on the back wall. It's not usually necessary to install an intake fan at the front, so long as the ventilation holes are kept free of dust
You can usually rely on the passive cooler on the graphics card to work well. Also if you go into the BIOS at start up you should find a page with fan speeds and temperatures but that shouldn't really be necessary