The answer is a little more complicated than the earlier answers here might lead you to believe.
It depends on which model of machine you have.
If you have the 2.66GHz machine 24", then I think the graphics chipset is part of the logic board. It also shares its video memory with main RAM.
If you have the 2.93GHz or 3.06GHz 24" machine than your machine had the option of three different video cards from the factory.
24-inch model with 2.93GHz processor, one of the following:
* NVIDIA GeForce GT 120 graphics processor with 256MB of GDDR3 memory
* NVIDIA GeForce GT 130 graphics processor with 512MB of GDDR3 memory
* ATI Radeon HD 4850 graphics processor with 512MB of GDDR3 memory
24-inch model with 3.06GHz processor, one of the following:
* NVIDIA GeForce GT 130 graphics processor with 512MB of GDDR3 memory
* ATI Radeon HD 4850 graphics processor with 512MB of GDDR3 memory
These are all separate cards, MXM cards, with separate video memory. They are essentially notebook video cards.
Finding a replacement, or upgrade card, is difficult as they are usually only available from Apple. eBay does have some and I found one on eBay. They are a chore to fit though.