Venturii have been used for a number of cooling applications through the years for one good reason: they cause the flow rate to increase _at_ the choke point. Unfortunately, they do not increase the total flow rate: Assuming your inlet and outlet are the same size, total in will be equal to (actually, because of friction, a little bit higher than) total out.
Basically, as air flows into the choke point (technically, this point is the venturi), its pressure increases. This causes it to heat up due to compression, but it _also_ causes its flow rate to increase (since outflow = inflow).
So here's what you do: Say you want to cool your CPU with a venturi. One way to do it would be to cut a large (120mm) hole in the bottom of your case. Make a venturi that scales down to an 80mm choke point; mount your fan in that choke point. Build the whole thing so that the fan mounts right below your heatsink (assuming you've got a vertical mount mobo). Then make a vent through the top of your case, et voila. This would really only increase cooling efficiency slightly, as venturii reduce turbulence and you would be drawing cooler air from the bottom of the case.
Alternatively, you could mount a 120mm fan at the bottom; this would push more air. According to the exhaustive cooling guide <a href=http://www.7volts.com/heatguide.htm><font color=blue>here,</font></a> the total gain from doubling CFM is only around 30%-40% more cooling, though, so you might want to think twice about that setup.
Another alternative is probably closest to the one your grandfather talked about: use the venturi to increase cooling on a water-cooling radiator. This is how a lot of older Army equipment is set up. This would be handy, say, for mounting a venturi/radiator outside of the computer, like in an auto stereo bass 'bazooka'. An added benefit is that is allows the use of large, low-RPM fans at lower noise than smaller, higher-speed fans.
The original uses of venturi have generally been replaced with compressor-driven cooling tech, but they still have the advantage of being simpler and less costly to manufacture. Current uses of venturi include aircraft (pitot tubes, ducted external radiators), some large ventilation fans, and pressure-reduction pipe couplings.
Here's rather detailed <a href=http://www.efunda.com/formulae/fluids/venturi_flowmeter.cfm><font color=blue>flowmeter calculator</font></a>, but you'll need to look up most of the inputs yourself.
"I laughed, I cried, I fell down, it changed my life . . . It was good!"