As long as he already has it might as well use the 9400GT for physx, no reason not to. It's not like it uses much power. IF you are asking if you should specifically buy a 9400GT for physx then no, I wouldn't bother.
not the power,I have enough
I just asking because I have a problem with noise , 2 cards will generate more noise
so is it worth it to have 470 + 9400 for physx ? or one is enough ? any charts about this ?
The 470 will provide enough noise to make the 9400GT sound non existing.. That means having both of them or just the 470 alone, noise wise its not gonna make any difference..
unless you play a few of the games on this list, don't worry about
the 9400GT won't really help much in the PhysX heavy games TBH, it might actually be slower than using the GTX470 for graphics and PhysX