I think I saw a quad core radiator once. Don't know if it's practicle though. Check ocforums.com and do a search. (they also have some sick applications of car radiators mounted to pc cases.) I just bought a BIP III for my rig and that should provide all the cooling I need.
A small radiator from an oil or transmission cooler would make a nice computer radiator for an out-of-case application. You should be able to pick one up at a salvage yard really cheap. Maybe use the core of an air conditioner (it has dense fins and lots of coils in the tubing). You just have to rig up some hose adaptors but that shouldn't be very hard, plenty of parts at any Home Depot or Lowes.