Airspirit,
A "throttling" valve isn't a specific type of valve, though I'm sure you've already found that out by now. It merely imposes a restriction that you can adjust. Look at it from an electronics point of view. At the point where you split the flow to go through the radiator and bypass line, the pressure will be the same. At the point the flows rejoin, the pressure will be the same. The valve is like a variable resistor. If you close the valve (infinite resistance), then all current goes through the radiator. If you open the valve fully, you will not have zero resistance, so a lot of flow will go through the bypass but some will still go through the radiator. What you want is a valve that is full port and stays in you position (percentage opening) you select.
To anyone doubting that slow flow through a radiator is bad, all you need to do is examine the extremes. At zero flow, there will be zero steady-state heat transfer. Slow flow is not the answer to better heat transfer in a radiator. Recognize that getting the water to the same temperature as the air is not the same thing as getting a lot of heat out of the water. Heat transferred is basically flow rate multiplied by delta-T in the water. Low flow means you need a heckuva lot of delta-T. Assuming you can get down to near ambient, this means that your inlet temperature (the stuff leaving your CPU) must get progressively warmer as the flowrate drops. As you increase flow rate, the delta-T in the fluid will decrease. In the real world, you reach a point where the added pump power required to get more flow through a radiator becomes counter-productive. Hence airspirit wanting the ability to artificially limit the flow rate. Honestly this approach will do very little to the effectiveness of the radiator, but by keeping the total resistance lower (combined resistance of rad + bypass line) the total system flow rate will be a little higher. This is a good thing for the blocks.
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