Well, there are alot of factors affecting radiator performance other than the length the water travels throught the radiator. One way the water can be made to stay in the radiator longer is by using larger passages through the radiator. If the passages throught the radiator are larger the water will flow more slowly throught the radiator. This also has the benefit of offering less restriction and therefore resulting in better overall flow and less load on the pump.
Probably the best radiator, in terms of performance, you can possible get for a watercooled PC is a heater core. This is a type of little radiator that is used in cars to provide heat to the passenger compartment. A certain amount of hot water from the engine is directed to the heater core and a fan blows air through it into the passenger compartment of the car. Anyway, heater cores make great radiators for watercooled PCs. They can also be obtained rather cheaply. I bought a heater core for a 1980 ford mustang from Autozone for less than $20. You may also be able to find used ones at an even cheaper price, but I'd just get a new one. Many of them are made of copper and therefore should be used with a copper waterblock. Although I have seen Aluminum ones as well. You might also be able to find one on ebay at a decent price. The only downside is their size and the fact that they may not be as visually appealing. Some of them might be just a wee bit larger than some of these other radiators that are often used for watercooling and therefore may have issues fitting into a computer case.
see this
thread
and this
one
Anyway, whatever radiator you use one thing to keep in mind is that if you use a copper waterblock you should use a copper radiator and if you use an aluminum waterblock you should use an aluminum radiator. If you mix copper and aluminum together in your watercooling system you will get corrosion and the aluminum will be slowly eaten away due to an electrochemical reaction.