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The current 27" iMac I see in the refurb store has the ATI HD 4850. The new iMacs have the HD 5750. Am I right in thinking that PS and LR really don't care too much about the graphics card, and that it's the processor and memory that matter?
Lightroom is at present not graphics card accelerated.

As for Photoshop CS4/CS5 and accelaration, there's this information:
http://kb2.adobe.com/cps/404/kb404898.html

I don't think picking a 4850 would noticeably impact Photoshop performance though, especially since Photoshop makes a marginal use of a graphics card that in itself is only marginally better.
 
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