Too late now. It IS the best answer
Clock rates etc don't mean much. The best indication of performance is performance.
Benchmarks show the actual performance of the 280x and 770 almost identical, with the 770 edging out here and there.
For the best, most reliable performance, get the best single card you can afford.
The 280x, 290 and 290x are the best single card options at their price points ATM.
Similarly priced Nv options are slower and or more expensive at this point in time.
SLI/XF changes things.
Performance never scales 100%, and having two cards doesn't use double the ram, so you are always losing out.
Historically, Nv cards perform better than AMD when used as a pair.
If you are set on using two cards, Nv will be a more reliable option.
Running 3 monitors or high resolution, id go for a single 290x, which has 4Gb.
A 780 Ti will be a little faster for ~$200 extra.