I doubt it. It might be close, but I doubt it will be faster. Im actually in a similar predicament at the moment myself. I also have a 5850 and a PC ready to take a second one. Im just waiting to see what the new cards are like, and just in case the 5850 drops a bit more in price.
I prefer having one card in my setup, two cards can be risky, like you might get micro-stuttering in games, and two cards use more power. But it's sooooo tempting.
TBH I'm running Crysis on enthusiast at 30fps @ 1920x1080 with ONE 5850, everything else runs really fast too, I might just save my money. I dont need another GPU in there! Maybe if I wanted to game on three monitors I could understand it. Metro 2033 with DOF effect on is pretty system hungry, but apart from that there is no need for an upgrade