Tri/quad fire or Physx?

if money isn't an issue you can get another 5970 for quadfire and then you can use a PCI card for physx instead of a PCI-express ...if you have an open PCI slot

they make PCI versions on the 9500GT, that way you will have the best visual experience possible for your given setup

you really only need a PCI-E 8x slot to run a physx card aswell, so if you have an open 8x slot that will be able to run a gts 250, 260 or even a gtx 275 most likely so you can have 2x 5970s and a gts 250 for physx and beast through batman and mirrors edge like its a joke

GTS 250 would run it way better than the 9500gt, for ht ebest physx to price ratio i'd pick up a 8800gt, its singleslot too

For one, a 5970 should be enough to run any game at the highest possible visual settings.
Yes, you could use a 9500 for Physx, but, it would not work very well.
Physx requires a bunch of SP's. A 9800gt is what I would recommend at the very least to run Physx, otherwise whenever a Physx event happens, performance will suffer greatly.

Really a 5970 should be sufficient, but if you want an upgrade I would suggest PhysX. A used 9600GT/GSO 384/9800GT(8800GT) would be sufficient. A dedicated card for PhysX would be far cheaper (~$50), so if it didn't give you much of an improvement you could either keep it as a spare, or sell it with a very minimal loss. The same thing can't be said about a second 5970