And the Most Confusing Post of the Week Award goes to...
Your introduction mentions deciding between a 2.0 DP PM G5 and an Intel iMac. Then you talk about your 450 DP PM G4. But your final question is between performance differences between 1.25 DP PM G4 & 1.8 SP PM G5.
I will comment that some aspects of a 1.25 DP PM G4 will be faster than the SP G5, only when multithreading is an issue. You can check out the Apple Hardware Photoshop thread in Macintosh Computers to see different speeds for different machines. My 1.25 DP PM G4 really moves in comparison to SP G5s. Again, only with multithreading enabled with the individual applications and operations.