I wish it were possible for me to measure air volume, in order to explore just how much air is getting recirculated with just one of these cards installed.
tools:
hobby knife, 2 toilet paper tubes, large enough sheet of cardboard to cover the side panel, 3-4" masking tape (scotch 2020/2040 series)
with the power supply removed to get your hand inside the case for quick marking
take a tube of toilet paper cardboard notch it to fit over the back end of the back end of video card exhaust port masking tape the m/b end shut and the edges you cut to fit over the end of the video card exhaust,
remove the side panel , use it as a template on your cardboard sheet, line up the cardboard cut out panel with the edges of the case, reach inside the power supply hole of the case with a short pencil (golf score card length aprox.) or micro sharpie, mark the tubes position (1/4 of a crescent atleast) on one side of the cardboard side panel, pull the side panel away and use the 2nd toilet paper tube to finish the crescent into a circle and cut out with an hobby knife. seal up the gaps around the edge of the case and cardboard side panel with the masking tape and place your anemomoeter at the exhaust of the tube.
http://www.instrumart.com/Product.aspx?ProductID=24205
crude, simple, fast, cheap solution, or you can deck out a nice piece of pvc tube and a plexiglass sheet and spend more time and money but more professional. for a visual with clear plexiglass you can use a smoke pencil like the kind used in home energy air leak audits or fire tests
http://www.buydraegersafety.com/DraegerAirCurrentTubes-CH25301.aspx
(or in my case a turkey bastor with smoke sucked up from a smoke bomb which is rather stinky)