HD 4850x2 is a beast . But
It has two fans and gets really hot while gaming.
Many buyers complain about fan noise and high temps.
So overall,HD5770,4870 are better deal than 4850x2 despite being a little weaker.
though you won't notice a difference at that resolution
I said HD 5770 cuz it is very similar to HD4870 in performance.
BTW which PSU do you have?