760 shouldnt be an issue for an Oc'd x4. the 770 maybe ull lose from fps but ur board does take fx cpus but a cheap solution would be the 6300 and an OC and all u gain there is rlly two extra cores and slightly better performance from the piledriver cores. It really matters on the games u want to play as more modern games run better off either faster cores or more cores.
My friends 1090t x6 at stock ran his 670 no prob, the 760 basically being a 670 and ur x4 is overclocked should be similar to a stock x6 other than the cores lol.
If ur gonna upgrade ur cpu later than id go for the 770 now and deal with the bottleneck until then, anyway the performance gains from the 7770 to a 770/760 u wont even realize the bottleneck.