Heres my 2cents. Far too often people over-spec their water cooling systems, people think that throwing money at the problem(of cooling) will make it better. When it comes to your system, I wouldn't be surprised if it could cool
*2* 4870X2's + i7 920@3.8-4.0.
Would it would be pushing it near the limit(the CFX + i7 example)? Perhaps, and will the temptures be a bit higher? Defenitly, but unless your WANT the absolute lowest tempurtures, you don't NEED a ridiculous amount of surface area. Sorry about the mini-rant.
To answer your question, no you do not need a 2nd Radiator, and yes, your system will run quite well on the 120.3 PA.