THe G5 chip is faster than the G4
Dual core processors are faster than single
1.2 GHz is faster than 800 MHz
now how MUCH difference this would make is entirely dependent on the software that is running. Different software use the hardware in different ways. A program that is highly disk-dependent will show less improvement than one that is highly CPU dependent
BTW there never was a dual core
iMac G5 or an iMac G5 running at 1.2 GHz so your scenario is imaginary