because,
even though the iMac has a G5 processor running at 2.1Ghz, the speed advantage looses its edge when up against dual processors. One processor doing one thing with the other doing something else, at the same time.
The purchase of an iMac G5 would give you marginally more speed, but I would see it as a backward step in the Mac world. Apple is moving its entire line over to the Intel processor which makes the G5, for long term use, not a good buy.
Adobe will be producing new software for the Intel powered Macs which would mean that programs like Photoshop will run native and be much faster than is current.
I would suggest that more RAM in the PM G5 would be more advantageous than buying an old iMac.
Just my opinion
P
I have the Power Mac g5 1.8 Dual Processor , 1.25 Gb RAM with Panther . I am a photographer and work with Photoshop CS2 to edit my photos. ( 200 to 300 per day ) Will the buy of a Imac G5 2.1 with 2.5 Gb RAM increase the perfomance comparing with the Apple i have now. What i need is more speed to do my job .
Thankas a lot
Luis Escobar

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