So where am I going with this. Apple ships and supports a limited set of ATI/AMD "Reference Design" cards in OS X, they do not provide support for all models or any major variations from the "Reference Design". The reason being that the ATI/AMD architecture requires specific model driver support. For example in 10.6 Apple only supports some 46xx and 48xx cards, there is no OOB support for the 45xx cards. So that is why you see patches and/or modifications for Apple's drivers to support some of these other cards.
The ATI/AMD 5xxx cards are finally easier to install. This has more to do with the GraphicsEnabler technology used within Hackintoshing, Kabyl's boot loader and ATY_Init. Even then you are limited to what cards are supported Out Of BOX (OOB) by Apple. For example the following are the only desktop ATI 5xxx cards Apple supports OOB:
ATI Radeon HD 5800 Series: Device ID 0x6898
ATI Radeon HD 5800 Series: Device ID 0x6899
ATI Radeon HD 5700 Series: Device ID 0x68B8
ATI Radeon HD 5600 Series: Device ID 0x68D8
ATI Radeon HD 5500 Series: Device ID 0x68D9
If your card's device id is not in this list, then you need to add your device ID to ATI5000Controller.kext and possibly ATIRadeonX3000.kext. This is great workaround but will be needed to be done after each and any Apple software update that touches the ATI drivers. Ouch.
NVIDIA cards use a different architecture and allows for a more generic driver than can support more models and variations. The new 4xx cards are based on a new architecture nicknamed "Fermi" and require new drivers. Check out this article on Tony's blog for our solutions for NVIDIA 4xx cards.
So until Apple ships support for more cards or allows AMD and NVIDIA to ship generic drivers, graphics card support is a challenge.