AFAIK PhysX needs a 9xxx or later card, but the point is moot anyway, the 8400 is far too weak for that use.
@ m2300: That PSU needs replacing if it's only 300W. For maximum compatibality you should aim for a unit with at least one 8 pin and one 6 pin PCI-E connectors (dual 8 pin for preference) and 550 to 600 Watts output. Stick to the better names: Corsair, Seasonic, Pc Power and Cooling, OCZ and Antec.
And to play Devils Advocate against Sunius
I'll suggest the HD7950, it's less expensive than the GTX670, but of similar performancce so you can get an uprated PSU and still stay under $400.
You may also find this article useful in making a decision:http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32571.html