Without knowing the rest of your pc /cpu , m/b, ram and your exact intentions its hard to give a recommendation. But I think I get what your saying.
I would move up to the 6850 if your cpu can make use of it, its MUCH faster especially when o/c than a 5770. You can get near 5850/5870 performance from a 6850 which is what 2-5770's would give you. Which is more impressive coming from a single card.