This place has everything you need:
http://www.xoxide.com/Using a car radiator really won't work unless you get a good pump rated at about 1/2 to 3/4 HP. A car radiator can hold from 1 and 1/2 gallons
to about 2 1/2 gallons of water. second problem is going from about a 1/4 inch tube from the cards to a 1 inch hose to the radiator. That is going to be a lot of back pressure created. For about $100 you can get about a 1 foot sq radiator that should work the way you want.
A friend of mine did the same thing you are trying to do, but with 4 cards, and still had to put a small radiator between the cards due to heat being transferred between the cards. The heat transfer of heat from the radiator to air is just as bad as using the stock fans on the card, unless you figure a way to blow it out a window or something.