Dedicated PhysX Card Question

Hey guys, I know this might sound kind of stupid, but I had a quick question regarding using a dedicated PhysX card. I had an old 9800GTX+ Black Edition (means it was overclocked by XFX) that I just upgraded to an MSI 560 ti Hawk. My original plan was to hang onto my old 9800 to give to my parents if/when I upgrade their computer, but when I went to launch Arkham City, I was looking at the settings and I noticed that it said under the PhysX section that to run on high, you should use a 570 GTX and a dedicated 460 GTX. Well this got me thinking as to whether or not it is possible to SLi my old 9800 GTX as a dedicated PhysX card. I know that normally when you SLi, you need 2 identical cards, this is why I was confused initially. When I googled to find some information about it, I found this article, which talks about independent PhysX GPU, which is the closest to what I think I would be doing, but I wanted to be sure that I'm not wasting my time or overworking my PSU for no reason by doing this. Thanks for the help!