I am running a 3-core copper brass radiator that I got from Radiator dot com for around $140. I have been very pleased with it so far. If there is enough air running through it you can watch the temp gauge drop.
Airflow is the single most important factor in engine cooling. The more air that flows through the raditor, the more heat it will dissipate.
The things that really matter on a radiator are total cooling tube and fin surface area and airflow efficiency through the radiator. The more tube surface area the more heat that can be exchanged. Some radiators have 3 rows of tubes that are smaller than stock and don't make for much added surface area. On the other hand, there are 2 row radiators with large wide tubes that have lots of surface area. One thing that is being done is called "tube dimpling", this is where the tube is dimpled like the surface of a golf ball. this dramatically increases the surface area of the tube without making the tube bigger, therefore increasing the cooling ability of the tube without hurting fluid flow.
Now, copper transfers a greater amount of heat than other metals. Aluminum, however, transfers heat faster and is less prone to corrosion under most conditions. Also it is much lighter than copper brass radiators. I wish the manufacturers had ratings for total cooling surface area or some kind of benchmarked cooling efficiency rating, it would make choosing a radiator much easier.