I don't think there are anything special with the Boxx machines. I have a 21 inch iMac G5 and it is perfect for the job. No problems what so ever.

I'm guessing the new iMacs are using the Geforce cards now; not ATI...

You can try this:

Go to /Users/[username]/Library/Preferences/Autodesk/maya/2008

Create a file (if it doesn't exist) called Maya.env

Enter these below into the file:

MAYA_SLOW_DRAWPIXELS = 1
MAYA_GEFORCE_SKIP_OVERLAY=1

Save and restart Maya.

Give that a shot.