Lightroom is at present not graphics card accelerated.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?
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.