swifty morgan
If you search google or youtube,you can find plenty of CPU physx benchmarks and videos.
https://www.youtube.com/watch?v=3WR6kb3oxDs
https://www.youtube.com/watch?v=uf2s4TdnH9w
https://www.youtube.com/watch?v=O_VE5yqNbLg
https://www.youtube.com/watch?v=N78L_iy7IZ8
https://www.youtube.com/watch?v=w2GblxK67nQ
I can find more if you want.
in addition, I'm running ati 5870 with i7-920 and I played many games with physx set medium including mafia II,batman arkham city,metro 2033,these games weren't playable at stock CPU speed (2.66GHz) so I had to overclock it to 3.33GHz to get 35 FPS, I agree that these games are three times faster on high-end nvidia graphics cards like 680/670 but you can get playable frame rates when you set physx to medium if you have fast processor.
bystander :
That depends on the game. Some are way to stressful to even attempt on the CPU, while others are not much different. Some give you advanced option without a PhysX card, some do not.
and about the physx option ,I read in article that some people needed a tweak to force the game to enable physx with AMD graphics card in arkham city and border land 2.
Quote from this page http://physxinfo.com/news/9425/borderlands-2-is-cpu-capable-of-handling-the-physx-effects/
(P.S. if PhysX option is grayed out on your system, you still can change the settings using "PhysXLevel" parameter in "WillowEngine.ini" file:
(\Documents\My Games\Borderlands 2\WillowGame\Config)
PhysXLevel=0 - PhysX Low
PhysXLevel=1 - PhysX Medium
PhysXLevel=2 - PhysX High )
but I didn't need a tweak when I enabled it in any of the games I played.