For some reason, Tiger still insists on representing your computer with an icon showing an iMac. It's not even a new iMac, and besides, I don't have an iMac. The file you need to replace is called iMac.icns, and, along with a lot of other system icons (like those for Desktop, Trash, Finder, Get Info, etc.), it's now migrated to this directory:
System -> Library -> CoreServices -> CoreTypes.bundle -> Contents -> Resources
Now you can replace any of these with .icns files of your choosing, so long as they have the same name as the original file. Go crazy...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050505000721260