crossfire my 5770 or buy better one?

i want to upgrade my gpu but here is my prob. I already have a 5770 and i want to know if i just add another 5770 if that will be worth it. Or should i just buy a 6850? i cant go any higher in price than that. Also, my 5770 is like 1 1/2 years old but it has been well taken care of. It has been in a well vented case with plenty of fans to keep it cool. Should i be worried about it going out soon? thanks

Crossfire.

sell it and get a 5850 ! it will last you for a good while then after a year you can upgrade