I would say replace the 770 with a 290. Here are some reasons why to consider the 290 over the 770.
Significantly more memory 4,096 MB vs 2,048 MB 2x more memory
Significantly higher pixel rate 60.6 GPixel/s vs 33.5 GPixel/s More than 80% higher pixel rate
Higher memory bandwidth 320 GB/s vs 224 GB/s Around 45% higher memory bandwidth
Better floating-point performance 4,800 GFLOPS vs 3,213 GFLOPS Around 50% better floating-point performance
Better 3DMark 11 graphics score 14,570 vs 11,310 Around 30% better 3DMark 11 graphics score
Significantly more energy-efficient idle power consumption 87W vs 109W More than 20% more energy-efficient idle power consumption
Significantly more render output processors 64 vs 32 Twice as many render...