ur current setup is fine, ur overclocked cpu is still a beast
for ati or for nvidia:http://pcpartpicker.com/part/evga-video-card-02gp42662kr
one of those will do u fine.
the 7870 at stock is very similar to the 660, dont spread false information.
the 7870 might overclock a bit better, but onced both overclocked theres a small gap between them, id say get amd if u like amd if not get the nvidia card, both are great for the money