As i said, main intention is to go from dual gpu to single gpu and STILL get a noticeable decrese in heat, sound and an noticeable increase in processing power . .
Then manufacture OC gtx580, with modded cooling > anything else

It will run stupidly cool, even when oced, and be silent as you want it. Belive me when I tell you my GTX580 3gb is not quiet or cool. When the cards get hot, I have to turn my speaker volume up lol.
Also one GTX580 (not oced) doesn't beat my old 5970, but it's close, I'm sure it will beat two 4870, but not by much at all. The oced version of the 580, matches the 5970 in some games like Crysis, and beats it to a pulp in everything else.
There is just no reason to get the 3gb version, it's not quiet or cool, it's made just for triplescreen gaming, which it's fantastic for and not what you are going to use it for!
If you honestly do want a faster, cooler, quieter card to replace your current cards, you wouldn't be serioisly concidering the 3gb version of the GTX580, when it does not fit any of your requirments.