Does your case have an intake fan low in front? If so, those hard drives could be blocking it a' bit. I'd also suggest trying to get those flat IDE cables out of the way of airflow. If not moveable, you can double fold them between the connectors and use the nylon bundle ties(lil' ones) to secure them. This would improve the airflow. Looks like you have an upper rear exhaust fan below the PSU and that's a good thing.
Looks like you have an AGP slot for video, so the best choice for installing a card type fan would be on the opposite side of the vid card from the side it's fan is on. If that's possible. With your other PCI slots,i'd put the sound card(if not onboard,that is)in slot #3 and your NIC or modem card in slot #4 or #5. The more breathing room the better for the vid card. Looking at you hard drives again,,might be better if you could move the top one up to the next available spot for more breathing room there also...