1) sell it if you can.
2) just keep it as collector item
3) as for dedicated PhysX i think it is better to let GTX660 alone to handle that. GTX260 use more electricity than GTX660 and to limit it's function solely for PhysX calculation (which is hardly any title will use it) i don't think it's worth it
Thanks you all for the quick replies. I thought this would be the case but I was looking for some confirmation.