If the second fan is installed at the opposite side of the case, then it should blow in the opposite direction of the PS fan. E.g. it the PS fan sucks air out, the second fan installed in the lower front part of the case should blow in. If both fans are sucking air out, then they would both try to create a vacuum inside the case, and there would be no real airflow. These suggestions might seem a bit trivial to you. However, I've seen many incorrectly installed fans, so these tips might still be useful.
Case fans come in many flavours.
If your case has a preparation for a secondary fan, make sure you buy a fan that has the right size for it. Otherwise, you'll need a fan with a standard power supply connector. Keep in mind that the higher the CFM, the higher the noise. You can find a large selection of case fans on SDS Mark II's heatsink page . At the time they were selling this unit, 3DfxCOOL said supplies were limited.