John or HJ -
Do you see any potential problem if I were to use 1" PEX for the main return and supplies lines instead of copper? This would be plumbed in a 2 pipe reverse return as John suggested. I have a very low basement ceiling so any space I can save is a plus.
On a side note, I think the 2 pipe system is the best for me. My radiators have independant shut off valves so a monoflow setup would not work if one radiator was closed. I'm also thinking of installing thermostatic valves for the radiators on the 2nd floor to control temperature up there. I cannot use these with a monoflow system.
Thanks again for the great advice.