A quick run through the PSU calc shows you should be OK as long as that PSU puts out as advertised. I am not familiar with that brand however, so I am not sure if it will. That said, if after you try running PhysX on your GT alone you think FPS are too low, I see no reason why you shouldn't try putting the GS in for PhysX. If due to old/poor quality the PSU is insufficient, you will probably have some random restarts/crashes. However, for those cards and assuming nothing is OC'ed, I think you probably should be fine.
Yep, that should be it I'd think. It should be fairly simple. The only extra thing I'd suggest doing is check NVidia's site and make sure you have the newest PhysX driver (I think each game that uses PhysX should update this, but no harm in checking.
I didnt see a BIG difference in running physX and not running physX, If you don't have a physX card it will run very smooth without physx on. If you do then it will run smooth with physX on. PhysX did NOT change the game play, just eye candy pretty much, Nothing to worry about getting if you don't have a physx card but if you have a 2nd video card, this is a good way to put it to use.