because in the uk the 5850 is £260 while you can get 2 5770's for £220 giving the same or even better performance.
I think the reason most would suggest spending the extra 40 pounds is that the 5770 is a dead end upgrade and you also inharit some draw backs of CF, which is that not all games support it.
With the 5850, you can add another down the road, when they are cheaper than today and add at least 50% more power, where as to upgrade the 5770 down the road, you'd have to dispose of both 5770's and start over.
Depends on the games, if you play modern games then CF support should be great, for the future, it makes sense that CF/SLI support should only be getting better/more widespread.
So it makes sense to go for the CF.
The upgrade thing is a problem, however the dual 5770's should perform great for at least 2 years.
however, im not sure if my power supply (corsair 550vx) will be able to deal with 2x 5850s, i think that might be stretching it a bit so thats why i have chosen some graphics cards that have low power but still provide good performance. but if any of you guys think that it would be worth upgrading the power supply and buying a 5850 instead of 2x 5770s then please let me know.