This is irrelevant, as you can't compare shader and memory clocks on two very different architectures. The 460m will be slightly slower in anything in the playable framerate range. It will win in heavy tessellation games, but we're talking 16fps vs 12fps there.
As for overclocking, the 6870m (5870m) is much more capable. Some users (including myself) were able to get an extra 35% out of the core at stock voltage. I was even able to overclock 20%(835MHz) on the core after undervolting the chip 7%.