Can I upgrade the Radeon Pro graphics card in my iMac early-2008?
Yes, yes you can (although it may involve a firmware upgrade):

http://www.ifixit.com/Guide/iMac+Intel+20-Inch+EMC+2133+and+2210+Video+Card+Replacement/1014

(Taken from: http://www.ifixit.com/Answers/View/90234/Can+I+change+my+graphics+card)