Can you Upgrade the 2008 Imac Graphics Card

Hi I purchased a 2008 24 inch iMac with a 3.06 intel processor and the Nvidia Geforce 8800 GS Graphics Card. Well Basically the graphics card has burned out and I am out of warranty and need to replace it. Took it to apple and they said they would replace the same card for about $500, eww... So then I thought if I have to replace I might as well upgrade. The Cards the newer imacs offer now are the Nvidia GT 120 or the Radeon 4850 HD and I was wondering if it was possible to put either of those into my 2008 Imac.