demon 2 x 5770's are not 50% better than a single 5850.
@ OP Go for a 5850, that way if you feel you need more powahhh then you can get another for CF.
the brand isn't to important just go for the best deal. I have a Gigabyte 5850, but I'm sure other brands would be just as good.
On a side note, I've just overclocked my HD5850 from stock 725/1000 to 920/1125mhz, on stock voltage
Excelent overclockers. Not that it needs to be overclocked :P