You'll have to play driver games to get PhysX to work while there's an ATi card installed in your system, but from various articles I've read, the weakest you might consider is a GT240 (if you can still find one). I don't know how the GT440 would do. The next step up would be a GTS450.
The cheapest is a 8800gts 320mb edition or a 512mb 9600gt. A gts 450 is more than most will need for physx for at least another year before those requirements are raised a little higher. Don't bother with a gt240 or a gt440 as they cost more than older cards that can offer much more bang for the dollar/pound. If it must be new then skip over the rest and go for the gts 450. It is better to have more than you need than to have less than you need only to suffer through the lag.
the 430 is a rebranded 9800gt which will work great with physx
I would have expected a 6 year old to say something like that but you don't know anything about cards. A GT430 is a Fermi era card not a rebranded G92. GF108 core not a G92 and I don't see how one could mistake the two as being the same unless wasted on drugs or alcohol.