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.