I'd consider 2xHD4770.
The HD4770 runs at roughly 90-95% (depending on the game) of the HD4850 but the power consumption is very, very low because of the new design and that it uses the new 40nm process.
You can look up power consumption in graphics card comparison at Tomshardware.
If you want more power then go with 2x4870 or 2x4890. They will be a lot more expensive and use a LOT more power thus being noiser.
The 2xHD4770 while not the fastest on the market does give very impressive results. I'm still running a 1xHD3870 and have determined that a single HD4770 would max out my X2-4800+ S939 CPU.