Radiator cooling

[a0chicken]

If I mount a 120 mm fan in the front of my case (No obstruction to intake side of fan) and have it blowing on a 6" x 7" heatercore, how much of a difference can I expect compared to using two 120mm in Push and pull?

Also, what is the concept behind a shroud? From what I understand it puts distance between the fan and the radiator to eliminate blindspots, or to push more air through the rad.
What if I have a 120mm push, and an 80mm pulling with a shroud in a 120mm to 80 mm style?
Like this:
http://img.photobucket.com/albums/v82/a0chicken/Radiator.gif
Edit: The 120 mm is 70 CFM, the 80 mm is 35 CFM.

[TyphooN]

just suck with a 120, it will do better than that config, why wouldnt u have 2 120's for push/pull? sucking with just 1 will outperform the setup you have drawn.

[a0chicken]

Alright, and how should I shroud the sucker fan? Like should I give an inch of distance and just connect the corners or what?