well i am using a gtx 680 too and up to now i didn't saw any physx rendering problem, but normally adding another card for that helps.
if your psu can handle both video cards (680 + 460) togheter, try it, but i don't think there will be much performance difference, if can sell that 460, that is probably the way to go. but if you have a good PSU try it first maybe you can win some FPS in games like Batman.
Thanks guys! Figured it would be futile but u never know till u ask... I have a silverstone strider gold 850 so I didn't think i should be too concerned about the PSU. Cooling might have become an issue with the 2 cards slammed together in there anyhow...?