the 4850x2 is more powerful...it has the power of a 5850 or a gtx285. Both cards will run at about the same temp unless you were looking at a 4890 with advanced cooling.
You can always compare performance by checking out toms video card charts.
Why is a 4890 about the same as a 4850x2? The 4850x2 will have double the working stream processors. Do clock speeds and GDDR5 memory make it that much better? I know that the 4890 has better cooling and takes less power, but wouldn't the 4850x2 have the power to justify it?