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Does Photoshop ustilise GPUs for processing? It used to be RAM and CPU dependent, if it can leverage CUDA then the 480 is the winner.
Regardless of whether more GFX RAM helps (although I think it does) the 480 is a better card anyway.
Thats only if you intend on playing at 2560x1440 resolution.
In your case you might be better of with a 6970 2Gb.
Has more ram, and is similar performance to a 480.
Otherwise if it must be a 480 or a 560 then 480 hands down.
but the latest version of photoshop uses cuda i believe, so therefore the 480 is by far the better card to get.