The HD4890 can handle physx, just not as well as the nVidia GTX260
Here is another snapshot comparing the physx engines
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As you can see, besides the fact I really need to find more reputable diagrams, the 4890 out performs the GTX260 in the 1920x1080 test without the physx or AA on, however with the physx on high and a little bit of AA, both cards suffer dramatically (Though not un-playable) with the GTX 260 out performing the 4890
Here is the same deal, though with 4xAA included
link
There are examples located at this site where they run more tests, but to save you time looking too hard into this particular source, the GTX out performs the 4890 in all the physx challenges, where as the 4890 out performs the GTX when not running the physx engine or AA.
Actually, here is one more
link
It all boils down to what type of gaming you want to do.
That said, I would like to hear what muffin man has to say, I know more about nVidia cards than I do Radeon, perhaps there is something I am missing. And Hopefully, he can provide us with some more reputable stats.
PS: Unfortunately I can't link that website because this forums page keep censoring the title lol. But it's on the pics in the links.