From Second Life Wiki
Identifying your graphics card
Getting updated drivers
Please refer to the following page:
Updating Macintosh drivers
Residents using Macintosh computers with built-in graphics made by ATI or NVIDIA generally are unable to update drivers, except when Apple issues updates. Use the Mac's built-in Software Update to check for and download the latest drivers from Apple.
Users with Macintosh computers such as PowerMac G4, G5, or Mac Pro systems that have been upgraded with third-party graphics cards may be able to download updated drivers from the card's manufacturer.
Using older drivers
Generally, it's best to update to the current drivers. On occasion Second Life runs better with an earlier version of the driver. You may be able to find information other Residents have discovered about specific cards and drivers on the SL Forums.