Well, I won't recommend Ageia PPU
1) PPU won't be supported in upcoming PhysX SDK 3.0, and is not supported by current SDK 2.8.3 already. So it will be absolutely useless for future games.
2) Compared to dedicated GPUs, even low-end, PPU performance isn't fascinating.
Batman Arkham Asylum and PPU
"While I'm playing during PhysX intense moments (lots of debris: papers, rubble, what not) framerates drop down to as low as 12fps from what I've seen"http://physxinfo.com/forum/index.php?topic=48.0
Most Dark Void effects just won't run on PPU, Sacred 2 and Cryostasis are laggy, perhaps only one modern GPU PhysX game that runs on PPU pretty well - Mirror's Edge
3) Similar to Hybrid PhysX setups, ATI + PPU setups aren't supportedhttp://physxinfo.com/news/338/ati-physx-ban-even-ageia-ppu-is-affected/
There were some modded drivers to counter this, but I dunno if they are working
Thus, PPU is fine only if you want to play old PhysX library (Cellfactor, GRAW 1, Infernal, Stoked Rider, etc), but not current PhysX games
On other hand, Hybrid PhysX
(ATI on Graphics + NV on PhysX) is pretty popular now, even available only through special mod. You can find instructions herehttp://physxinfo.com/news/942/hybrid-physx-mod-1-02-195-xx-drivers-and-win-xp-support/
Hybrid PhysX benchmarkshttp://www.fudzilla.com/content/view/17525/1/http://www.pcgameshardware.de/aid,701321/Physx-Test-Radeon-dank-Physik-Geforce-bis-zu-7-Mal-schneller/Grafikkarte/Test/http://www.coolaler.com/showthread.php?p=2472931#post2472931
and many others through the web
also, there were a lot of articles about PhysX performance in Bataman AAhttp://physxinfo.com/news/244/batman-arkham-asylum-physx-benchmarks-roundup/
may be helpfull