Ageia Physx was CPU based from the get go. The PPUs were a tie in but not required. Still nVidia's gimping has been documented several times. Basically physx on CPU runs in x87, a set of instructions that no modern software has used since 10 years ago. x87 is the lowest common denominator but I'd say that whomever wanted to play a physx enabled game would have a CPU with SSE2 support at least.