First if all colder then water? Water isn't naturally cool, it's temperature is determined by the ambient temp of the environment the water is in and the technique used to cool it (if applicable.).
Anyway, here's my second shot at reorganizing this post.
From your posts it sounds like you're trying to mix two, completely different forms of cooling.
Any liquified gas such as LN2 or liquid CO2 wouldn't work at all in a standard water cooling loop. The first reason is that almost, if not all, liquid gases are WAAAAAAAAY to cold for standard parts to handle. The second is that, well, it's impossible. For you to understand why it's impossible you have to know how water cooling and LN2/dice cooling work.
Water cooling uses conduction cooling to cool a pc. LN2, dice, ect., use evaporative cooling.
The principles of evaporative cooling are a little hard to understand, but I'm pretty sure I can explain it. For a liquid to turn into a gas it requires energy, but it can't just create energy, since energy can be neither created nor destoryed, just transfered. So it has to get it from somewhere else, in this case, from heat energy. When using evaporative cooling to cool a pc the heat source is whatever component you're trying to cool, the cpu, gpu, whatever. Energy is transfered from the heat source to the liquid. Less energy = less heat. When the liquid has enough energy it turns into a gas. Then both the gas and the transfered energy become a part of the atmosphere, removing them from the pc completely. The colder a liquid is, the more energy it requires to turn into a gas.
Another example of evaporative cooling us the creation of dry ice. Liquid CO2 cant exist at regular atmospheric pressure. That's why it turns from a solid to a gas. All you need to make dry ice is a tank of liquid CO2. When the tank is depressurized the liquid immediately evaporates, this lowers the temp of the liquid CO2 that hasnt vaporized, since the evaporating liquid needs energy, creating ice. The ice is then packed into pellets, blocks, ect., and shipped out.
Conduction cooling does just that, conducts heat. In a water cooling loop cold liquid starts by running across the hot components, the water absorbs and thus removes the heat from the components, cooling them. The now hot water moves to the cool radiator, where the heat energy is transfered yet again, then removed from the pc and into the atmosphere through the use of fans. So the water cools the components, the radiator cools the water, and the fans remove the heat. Same end for heat energy, just a different way of getting there.
The best fluid to use in water cooling would be an anti-freeze/water mix, possibly with some water wetter thrown in. Water has one of the highest specific heat capacity of all liquids, the only thing I can find that's better is liquid ammonia, and that would be insanly hard to use safely and effectively in a wc loop. Specific heat capacity is the amount of energy it take to raise the temp of an element by 1ºc. The higher the specific heat capacity, the more energy an element can absorb before it's temperature is raised. The water wetter and anti-freeze pretty much do one thing, heighten the boiling point and lower the freezing point, essentially raising specific heat capacity of the liquid.
So now that the basics are covered, I can explain WHY it's impossible.
A water cooling loop is closed, meaning that the only way to expel heat is from conduction cooling, since there are no exits for gases and the energy they contain to escape, so energy can't be removed from the system. To turn LN2 (or a liquid gas like it) from a gas into a liquid you need extreme cold. Regular conduction cooling can't get below the normally hot (when compared to LN2) ambient temp of the room, because it expels energy into the atmosphere, and there's a certain amount of energy already in the atmosphere. As I said before, energy can be neither created nor destroyed, only transfered. If the gas cant get to the necessary temp for it to turn into a liquid, the evaporative cooling process cant start over again, and you end up with a WC loop filled with gaseous nitrogen.
If you really want to go for the gusto here is a how to, but be prepared to get your wallet out. Note the video download on page three it may be old but it's still fun to watch.
) you'll have to raise the pressure to 5.1 atmospheres and cool it to -65 degrees Fahrenheit. If you raise the pressure to about 100 atmospheres it'll liquify at +88 degrees Fahrenheit. 
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