Which GPU is best for my Nvidia PhysX

The 8800GTS.
The GT 210 is entry level junk.

8800gts 320, why because that 210 likely isn't even going to meet minimum requirements for physx as that requires that the target card have 32 or more shaders. Most 210 have only 16 with a few having up to 24. if your system can't handle the power load of the G80 card then start looking for a less demanding card like a cheap used 8800/9800gt while a gt240 will be more than enough except for a few rare instances.

Then ASUS 8800GTS 320mb can do the job without any faults?

Good article on the subject:
http://www.tomshardware.com/reviews/nvidia-physx-hack-a...

If you've already got the 8800 use it, it will definatly make a difference. That article shows that at least in Mafia II though that there is plenty of physx processing to be done and that there is benifit to having an even higher end card for dedicated physx.

You will be amazed at the gains in Batman even in a high end system by having a dedicated card. It is the way go knowing how weak cpus are when it comes to physx and how demanding new physx games are getting. A dated ageia ppu still beats an overclocked i7 in this area.