Rafiki

think of it this way,lets say after passing over your processor,the coolant is say 20 deg. above ambient.
then the rad/fan combo only cools it to 10 above ambient.
if you were to pass the coolant through another rad/fan combo before it returns to the processor,you could scrub more heat from the coolant thereby getting it closer,if not at,ambient temp.
although without any other means(peltier,phase change add-on etc.) the best you can hope for is to get the coolant to ambient temp.
edit: (IMHO. if your coolant isnt reaching ambient temp BEFORE it goes back in to the processor,you have an overheating problem waiting to happen.)
then again,im no W/C pimp.
.02