Louis Wu at HBO, tipped off by Toxicoman and Howie, posted that there are system requirements for Mac Halo listed at the Apple Store. Look on the right hand side, in the "New to the Store" column, or do a search for "Halo". Nevermind that MacSoft's Al Schilling said only yesterday they weren't determined yet. Here's what the Store says:

• Power PC G4 700 Mhz or faster
• Mac OS X 10.2.5
• 256MB RAM (iMacs req. 512MB)
• Radeon 8500 or Geforce 2 MX with 32MB

A couple of interesting notes there. Despite the much-vaunted "Megahertz Myth", and the difference between OpenGL and DirectX9, the requirements for PC and Mac Halo are nearly identical. That says to me that either the video card is more important to Halo than anything else, or these system requirements are really just a copy of the PC requirements.