Here's a useful link:
The minimum processor required to effectively run Adobe CS4 for Mac differs according to the version of CS4 that is being installed. Basic versions such as CS4 Design Standard can use either a PowerPC G5 processor or any of the Intel multicore processors. More advanced versions, including the CS4 Master Suite, are unable to use the PowerPC G5 and require an Intel multicore processor.
I would go for a second hand Intel if you can, as the door is basically shut on PowerPCs.