I ran a few benchmarks last night (Heaven 2.1, AVP, Dirt2, Stone Giant and Metro 2033) and the only physx one i have is Metro 2033. I installed the latest nvidia drivers and took turns selecting the 570 and 8800GTS as the physx card in nvidia control panel (both running at 8x PCIE 2.0).

I was only seeing a difference of 1-2 fps in min and avg framerates in Metro 2033 (no difference in other games obviously) compared to using the 570 for both GPU and physx. Max usage on the physx card was only about 15% whilst the 570 was at 96%+!! Is this right or do i need to do something to get physx working properly?

Seems like it might not be worth keeping the 8800 as anything other than a backup unless I'm doing something wrong.

Your 570 dont need any help.

Metro 2033 doesn't really make that much use of PhysX not enough to need anything extra over a 570.

If it's the older G80 core then it doesn't support CUDA and physX.

If it's the newer G92 core (Pretty sure it is) then something is amiss surely?

Only game I know that makes 'HEAVY' use of Physx is Mafia 2.

The physx requirments for that are steap!

See if there is a demo you can get maybe?

Well as physx is only in Metro of the games that I play i'll just leave the 8800 in the drawer until something that needs it comes out, maybe GRAW3.

Cant see as it's worth the extra power and heat for the sake of 1-2 fps unless something is configured wrongly.

It's a shame u cant push other things onto the second card other than physx, a dedicated tessellation card would surely make a big difference in DX11...

The latency involved in sending tessellation data off card and back would kill performance.