The GTX 460 will make a great PhysX card and will pair up nicely with a GTX 660 Ti.
A couple questions you need to answer:
- Can your PSU handle two cards? - I'm estimating 650 watts minimum (luckily the 660 Ti is very energy lite)
- Do you intend to actually play any GPU accelerated PhysX games?
Here's a list for reference: http://en.wikipedia.org/wiki/PhysX
If the answer is yes to both questions, go for it! Heck for $50, I'd go for it anyway.