It improves the performance and reliability of graphics-intensive games and applications and fixes an issue that some customers encountered when installing Mac OS X Leopard after applying iMac Software Update 1.2.
For further information on this Update, please visit this website:
Apple iMac Software Update 1.3 (Leopard)
This update provides important bug fixes and is recommended for 20-inch and 24-inch iMac models with 2.0, 2.4, or 2.8GHz processors.
For further information on this Update, please visit this website: http://www.info.apple.com/kbnum/n306851