The PPC970FX is 90nm, the original PPC970 was 120nm.
Right now the PowerMac will use the PPC970 whenever the supply of PPC970FX is constrained -- this keeps the iMac G5 production line from being idled.
And yes, they have used some 120nm chips in the new PowerMacs since they did the switch to 90nm -- since some people were rumored to have ended up with them in new computers built after Aug/Sept of last year (or whatever the date for the 90nm switchover was, you'd need to search for it.)
The iMac G5 (and new SP PowerMac G5 1.8) can only use the 90nm CPU, the DP PowerMac 1.8/2.0 can use either the 90nm or 120nm while the 2.5 is 90nm only.