The 770 starts at 1046 Mhz boosting to 1085 Mhz by default. The best card I can find is the Gigabyte OC which takes that to 1137 and 1189.
In comparison the 280X starts at 850Mhz, is roughly the same speed as the 770 (according to gamegpu.ru data across a wide range of games, GE comes in slight faster, 7970 slower, 280X should with linear scaling also be very slightly slower but its like 1% so a tie). But here we have a card overclocked to 1100/1150. that is a much bigger increase in clockspeed than any of the 770's can muster when overclocked, it will clearly outperform the GE and any of the OCed GeForces. Its not even going to be a small difference the 280X toxic is going to end up a good 20-25% quicker on average. That is nearly enough of a boost to make sure that its quicker in every game (the swing on gamegpu.ru is 30%).
But there are other considerations on these two cards. PhysX, gsync (future) on one side and mantle (future) on the other. Then there is your perception of driver quality and the types of bugs that are a problem to you, differences in volume of the coolers and power consumption. Lots of other considerations that might swing you one way or the other on these card choices.