I'd have to agree with bill on that one. Even a silent fan that barely moves air helps, consider it. The best way to do it (in my mind) is just get fans quieter than the hard drive. Now I think the rule with todays processors is you want the pelt to be about twice the wattage so on a 95 watt CPU you probably want a 200 watt pelt. But keep in mind that now you have near 300 watts of heat to remove. I suppose with a couple large heater cores / radiators it may be possible but it would be terry hard to cool several peltiers and the computer passively. Another thing, your electric bill is about to get a lot higher
Now all of what I just said is if you want to slap the pelt right on the processor. If you want to build a chiller then you can probably use low wattage peltiers and passively cool them with large heat sinks. Now I'm not sure exactly what you mean by the formula for the prefect peltier but if you happen to be cooling an intel processor, head over to their website because they tell the heat output of the processor. Then as I have been told (but may be wrong) you want a peltier of twice the processors wattage to go right on the processor. But a pelt of any wattage can help water cooling but remember if you don't cover that however many watts of heat you have then you still must have a radiator. Also in case you were wondering I strongly recommend a pump.