Generally cheap-ish watercooling isn't really much beter than air cooling, BUT if you mount the rad outside the case, the heat won't be exhausted into the case, and in theoory case temps should go down, and possibly it'll improve cooling too (I would assume room temps are higher than case temps).
That's my theory anyway (though it will be a bit noisier).
A 120mm rad is a must, even 2x80mm gives less radiator surface area than 1 120mm rad, and the 120mm fan could probably be made a lot quieter than an 80mm fan (or two) if you want low noise with decent CFM.