Change the icon
By default, "About This Mac" will display a generic icon of whatever Mac model your Hackintosh is pretending to be. However, you can change this by replacing the icon with a different one. To find these icons in your own Mac OS X file system, open Finder, and go to:
NOTE: In the CoreServices folder, you will have to right-click the file "CoreTypes.bundle" and click "Show Package Contents".
Changing the name
By default, "About This Mac" calls your Hackintosh a Mac. For instance, if you have the Mac Pro (3,1) system definition installed, "About This Mac" will say "Mac Pro Early 2008". You can change this by editing a properties list (plist) inside Mac OS X. First off, download TextWrangler. The plist file that you need to edit for this cannot be rendered properly in TextEdit or most other text editors.
Next, enable showing of hidden files. Then, open Finder, and go to:
NOTE: "Your User" represents the name of your user account in Mac OS X. To open hidden folders, you may have to right-click the folder and click "Open".
Save the file, and log out. You're done!
Once you log back in, click the Apple on the upper left-hand corner of your screen, click "About This Mac", and then click "More Info". You will now see your new icon and system name displayed. Hooray!
SOURCE: How to Mod About This Mac for Mountain Lion! [tonymacx86]