Danger Den and BE cooling, the two arguably most trusted sites both consider stacked tranny coolers to be very good at cooling in the 12x5 size. Stacked coolers tend to have 30% better capacity to cool than tube and fin at the same size, but aren't half as good as radiator core types, but it depends what you need. I still fear aluminum w/ copper.
If your cooling a Duron 750@750, a 6x5 tube&fin compared to a Big Momma won't be any different. Now cool a T-Bird 1400@1700 with one or two 156w peltiers, and you'll surely fry with the 6x5 tube&fin.
What are your goals... What kind of CPU are you running and how far are you ready to push it? If you just want the biggest get a 4 core radiator from a lincoln continental, $200, 2 feet x 2 feet, but seriously, it depends what you need
What are you running and do you plan to use peltiers, or want the option?