I've seen a few people ask if you can pair a Nvidia card with an AMD card for PsyX, the answer to that is yes, yes you can. You'll need hybrid drivers but it can be done.
Now, I've never done it myself so I will post a link to were the drivers can be found, it's on a forum and the guy still updates everything so if you need help you can either search that thread or join and ask people who have solid experience with doing this.
A quote from his post:
Nobody before believed PhysX v9.11.1107+ could be hybridized:
But Now Works with all nVidia Drivers including the current(9.13.0725) and future versions(not v3+) in WinXP/7/8+ (Vista not possible) on 32 OR 64bit games/OS in PC systems containing both RaDeON & GeForce 8000-600+ series cards with 256MB+ VRAM! (or for IGP, allocated UMA RAM) This means both brands are in your machine, separate GPU&PPU. Any GTX260+ will be ultrafast, but whatever you have lying around should be sufficient for most games.
If you're interested in trying this out, all the info you need can be found here:
http://www.ngohq.com/graphic-cards/22136-how-to-fix-hybrid-physx-with-latest-physx-and-geforce-285-solved.html
I've seen this but could never justify cost, power, or heat.
why would you? you can use phsyX from your cpu
There was talk of this a month back with someone looking at upgrading to AMD but wanting to keep his old Nvidia card. Look. It can be done but there are a few things that really make it not worth doing.
1. Very few games use Physx. Its a fact and if you do play games that do use Physx then good for you. You get a massive boost in FPS as your CPU or Main GPU has more room to do what they need to do. Now there are games out there, Some of them are some of the most popular games played today or in the last 2 years but their limited in range.
2. You are waiting on 2 different drivers and then waiting for someone to bring them together. While it is being done, you will be waiting a long time just to use the drivers unless you have the ability yourself to complete the task. I would rather use the latest beta drivers available than wait 3 weeks for a modified driver just for another 15 FPS. When often with driver updates you get a 5-20% boost in FPS anyway as games and drivers mature.
3. Unknown errors and conflicts will lead to headaces in the future. Many professional softwares that say use OpenCL to improve render times or you are using will attempt to use all the graphics cards in the system. Adobe, Autodesk, Folding@Home. This usually is not an issue if you have say a Tesla, a Quadro and a GTX 650 Ti Boost. But when you start matching cards from different manufacturers with modified drivers. you are just asking for trouble.
Now im going to bring up consoles because this is something consoles will have an effect on. Both the Xbox One and the Play Station 4 have shown some form of accelerated physics computing. Its nothing amazing compared to PC but The consoles both use the same APU and that APU is of the same family as the R9 290X from AMD. Now this is just assumptions but I think there will be some new Physics accelerated calculations when Mantel comes out. Most likely AMD's answer to Physx and yet it will most likely boost the support for Physx as well. Just something to keep in mind.
Honestly I would not recommend people to do this. If you already have a Nvidia card, you want Physx and want to upgrade. Stick with Nvidia. If you want to take a gamble on what I just mentioned, Go AMD. If you dont really care. Chose whatever suites you and your budget. It really is up to you and what you want in your system.
I tried it out a while back, and it does work, but it is like Rudster says, you will run in to issues and problems. I walked away from it when the AMD card died on me, the memory got corrupted, and I got an NVIDIA card as a replacement. In general I would recommend in stead that people just stay with NVIDIA if physx is your thing and keep your old NVIDIA card when you upgrade. You can run the older card as a secondary dedicated physx card which is a much better solution.
Yes it works, but indeed you will also run into a lot of headace´s
If you realy care that much about PhsyX then i would say just stay at Nvidia ship. and just upgrade your current card to a better one.