Though I question whether getting a GT640 for a PhysX card is a good move. A low end PhysX card can drag down performance compared to just letting the main card to the work. http://www.youtube.com/watch?v=cbww3dhzK0M
Without doubt it will be better to have another 670 to SLI, but then why not do that in the first place?
Was just saying that you have to get a decently powerful PhysX card so it doesnt bottleneck the main GPU (estimate a card like the 650ti or 560ti). I personally dont see the point of having a dedicated PhysX card outright, too few games use it to really be a concern IMO.
Depends if the PSU in question has all the connectors you need. You should be able to run two cards off a 620W, but you will be running the PSU quite hard.