If you're playing games on your PCs, the spare cards could be used for PhysX provided the motherboards support dual cards and the powersupplies can handle the load.
OR you could keep one as a spare, new hardware can go wrong and it's helpful to have a spare for diagnostic purposes and then you could gift/sell the other to a friend/workmate who would appreciate it- a great way to sweeten up the boss prior to asking for a raise?
And there's always the great and wonderful E-bay, just do n't expect to get a new Porsche with the proceeds.
And do n't forget the small ads.
Upgrading from GTX 460 to a 560 Ti is just 2 jumps in the GPU ladder. Recommendation for an upgrade to be worthwhile is to get at least 3 ladders jump: http://www.tomshardware.com/reviews/gaming-graphics-car...
If your motherboard have 2 PCIe slots that is at least x8 x8, it might be more cost effective if you just buy a 2nd GTX 460 for SLI & maximize your existing card rather than just hand it down to your daughter. After all, she already got a decent GPU unless she is also a hardcore 3D gamer. But when doing SLI, please be aware of some peoples concern about microstuttering.
The GTX 260 is equivalent to today's GTX 550 Ti. It is a pretty decent GPU & can be used as Physx cards if you play games & your motherboard have 2 PCIe slots.
I would wait untill the 660 ti comes out and it is right around the corner and it would be your best bet for your upgrade. The physx is not all what is cracked up to be. You will never see the difference in doing that.
The thing you have to look at is your PSU's. Is your daughter's rig able to handle the 460? I am sure that your rig would handle the 660 ti because is uses less power than the 560 ti.
Anyway just though to give my 2 cents worth and what ever you do is good. I wish you guys good luck on your upgrades when ever that migh happen. As for your other cards sell them off to your buddys or family.