Can you change the processor from PowerPC to Intel?

No.

No, completely different architeture - you would need to completely replace the logic board and other components like video subsystems and so forth.

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It's not possible. Intel-based Macs use completely different logic boards than PowerPC-based Macs because chips are different (Intel-based Macs use EFI instead of Open Firmware, or PRAM instead of NVRAM), so if you want a Intel processor on your computer, you need a new iMac