upgrading to i5

Photoslop can be handled by either machine well [except for the poor display resolution, which when working on larger sized images can be very very annoying]
The i5 will handle larger file sizes better.
I'd hold out to next year if your main use is Photoslop though/ may as well hold out for a redesign and Ivy Bridge CPU's.

 

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Don't think Ivy Bridge is going to be
Magic, better then sandy bridge? Yes. But going to remake your computer? No.
Ivy Bridge is mostly working on making Sandy Bridge require less voltage, battery and help keep your computer cooler.

 

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Photoshop is CPU intensive, so you will see a large increase with the i5. Should be up to 80% faster which will be especially noticed when doing CPU intensive operations