Running peltiers can be done either with current or with voltage, but it turns out to be the same thing because (although they are non-ohmic devices) controlling voltage means that you control the current.
Having said that, you should focus on controlling voltage because that is what all the specs are based upon. As a rule of thumb, run the pelts at 75% of rated max voltage for optimum efficiency. Running more electricity through the peltiers will generate more waste heat to be dumped, but it won't cool the CPU any better. Of course, a peltier manufacturer will have a voltage-curve that tells where the peak efficiency is, but 75% is a good starting estimate.
Also, don't be misled by the max delta-T spec given by peltier manufacturers. The actual temperature differential between hot and cold side diminishes as more heat needs to be moved. It is just like an car that slows down as you go up a steeper hill.
BTW, peltiers only work down to about -60C. Electron mobility (which goes down as temperature goes down) in the current designs limits their effectiveness. This (along with some other reasons) is why some chips actually start failing to work at low temperatures. On a side note, I just read about how copper vias in modern 0.13um chip designs are starting to pull away from the M1 contacts because of thermal expansion and contraction. Temperature is important, and you can't just assume that things will work well at ultra-low temps.
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