Which GPU for dedicated Physx?

[!TIMMY!]

Ok, so I have a 8800GT and want to pop in a cheap Nvidia card to do all the Phsyx work in a multi-gpu set up (and the 8800GT will do the game). I can't find any reviews that compare how different cards handle physx when paired with the same more powerful card.

So what is the worst card I could get away with just for doing physx?

I have been looking at the 9500GT 256mb as it has a silent cooling solution.

 

[!TIMMY!]

hmmm, im also looking at the 8800GT 256mb as a possibility, but not sure if this will be a waste of £50 as Wizardskills said.