It depends on the games you play... Skyrim & GTA 4 are CPU intensive while most others are GPU intensive.
So it shouldn't matter which GPU you buy, because both 7770 and 7850 are very capable of playing games at 1366 x 768.
Since BF4 and Watch Dogs are next - gen games, there are only scant details on how playable they will be on those cards. You should be able to play them at medium - high settings on the 7770 and 7850 at your resolution.