K,
As you know, I have used watercooling (the infamous AquariusII kit, to be exact), but have switched back to air for the time being.
A very popular radiator seems to be the heatercore from an 86 Chevy Chevette.
It's a good size and has 1/2" fittings- the preferred size for serious watercooling systems it seems.
Of course you still run into the dreaded noise issue...a big radiator and an effective waterblock demand some serious airflow to remove the heat.
Twin 120mm fans (typically Tornadoes) seem to be the solution of choice.