Dedicated PhysX hardware

no,u cant mix nvidia and amd cards together...it will not work

There actually is a way, the cards wont work together in SLI or Crossfire or anything like that. But there is a method for running the nvidia card for physx. Someone recently asked the same question and were directed to a modified driver that allows this. You should note though that it will probably be a very buddy experience without a lot of support. Have a scroll down and im sure you will find it, it was only in the last day or two.

http://physxinfo.com/wiki/Hybrid_PhysX

only via hacked driver. as such, while it works in most cases, do keep your expectations low as it's not 100% perfect

do you mean this, burgies?

http://www.tomshardware.com/forum/370589-33-physx-card

note that he is running on a 530W PSU from Raidmax

You can run the 2 together.
Yes, you will need modded drivers for the rig, since Nvidia is certainly not happy with the ATI/AMD idea running so close to it....
I tried this with my Ageia PhysX card, then with other Nvidia cards.... and believe me, it's very easy to say it'll work.... but..... it's a pain in the butt getting it to work. And when it does..... that is for a very few games that support PhysX..... it doesn't really look that astonishing.... it looks fabulous with the demos and benchmark programs but , in game, it's quiet a waste of power and time ....
Plus the complications of having to look for the right drivers and mods every-time there is an update available really removes all the joy from the result.....

Thanks for the fast replies guys! I'm kinda sad that nVidia ruins the fun for everyone  
I might try it out but as I read these forums it doesn't really worth it.

Thanks again guys!
Khemist

By the way, whats the worse thing that can happen if my power supply cant keep up with the two cards?

It can work, in fact that my rig using it 5770CFX + 9500GT. But don't keep hopes up, I get to know it was quite useless. (for me at least)

I'm using it just for curiosity / learning sake...

It can work but if your psu is junk then I suggest that you replace it before it causes any problems. The good news is that the 55nm G92 isn't a power hog and better yet you have the older model before the crummy GTS250 came out so you got the best board that supported the 65nm model while allowing for higher clocks. You can use riva tuner to unlock the core from the shader clocks then push the shader easily to 2ghz. You can get more but that depends on the card and if you downclocked the core.

One more thing. Does underclocking my nVidia card gives me less PhysX processing power?

Underclock the core after you unlinked the clocks in riva tuner for that 9800gtx+ then you will have more room for higher shader clocks.

One more thing. Why would I want to get higher shader clocks? The whole rendering would be done by the ATI card and the nVidia card would only do the PhysX part

You can get a boost and it is best to learn this trick down the line when physx games become more demanding. Right now for almost all the physx titles it is just fine as it is.

Thanks guys! You all have been very helpful! Cheers!