Unless you really want lower power consumption, don't bother. The 7770 is actually a downgrade from a 5850. You might be able to get the 7770 to be a little faster than your 5850, but only if you can achieve a good overclock, which isn't guaranteed.
If you really want an upgrade, you should be looking at a 7870 at minimum. Even then, it likely won't make a real difference over your 5850 at your resolution, the 7870 is really meant for 1080p gaming.