"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 memoryNVIDIA GeForce GT 130 graphics processor with 512MB of GDDR3 memoryATI 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 memoryATI 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."
"As I was troubled by the conflicting information on this page/site, I wanted to testify I did just complete replacing the video card in my A1225 / EMC2267 24"" iMac. I bought the Mac new from Apple in 2009 with the ATI Radeon 4850 HD video card upgrade. After 4 years, the VRAM died...the picture would scramble at the logon screen unless I booted into safe mode. In safe mode, screen covered in blue dots, TechTool Pro confirmed bad VRAM.
I purchased a new card from Impact Computers in Florida. I had to fully remove the logic board to get at the video card, but with only three more screws it pulled free from the MXM connector, heat sink and all.
I've got pictures of the whole process if anyone's interested.
I'm confident one could upgrade the nVidia GT120 or 130 mid-line machines to the ATI card the same way. It may be the baseline machines (9400M graphics) don't have the MXM socket installed on the motherboard and are non-upgradeable."
"You can upgrade or add a discrete graphics card, so long as it would fit the case.. In terms of MB, any amount can be used, depending on the graphics card that you will use. The ""MB"" in graphics cards means that it has its own RAM that it can use, other than the RAM in your motherboard.