How do I make sure I purchase the correct replacement gfx card for my iMac? I have a 24" Early 2009 iMac, and it has a faulty graphics card (detailed here). I'm obviously no longer under warranty or applecare, and I'd like to try to replace the part myself.

I'm unsure about how to search for and select a replacement part, however, especially since my machine is quite old so the original gfx card model is probably not available new. How can I know that any given card listed on a hardware website will work as a replacement?

(A short list of appropriate gfx card model numbers would also suffice.)
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/