Neither is a perfect option. I have a 7770 and it runs most games, but will struggle at 1080p.
If you are gaming, then a better graphics card will usually be the better option, however the i3 might bottleneck your PC, so I suggest the i5. Maybe save up for a better graphics card.
The i5 will be more futureproof and won't bottlkeck your system in many years, but the 7770 or i3 will, so it is a tough decision.
I would go with i5 now, not get a graphics card for a bit then buy one later.
Ps the 280x is a lot better than 760.