If you buy an FX8320 and expect to use Mantle with your GTX770, you are in for a big surprise. You can use mantel with the i5 and a R9 280.
but Mantle just might give the FXs the edge
Doubtful as well. The 8320/8350 is a strong CPU, so the boost from Mantle won't be much. It provides a big boost to lower end CPUs like the 4300, etc. For me it depends on what you are building for, and/or how often you upgrade. i5 is better now. FX might be better once we program games to use more core. If you upgrade often then get the i5. If you upgrade very rarely, then the FX is the better idea.