How do I make sure I purchase the correct replacement gfx card for my iMac?
The NVIDIA GeForce 8800 GS is not available anymore, but you can try the NVIDIA GeForce GT 680, which you can find here: http://www.evga.com/Products/Product.aspx?pn=02G-P4-3682-KR
You will also need to upgrade your RAM to support this card. The RAM I suggest is here: http://eshop.macsales.com/item/Other%20World%20Computing/8566DDR3S4GP/