2.0, 2.4, or 2.8GHz 20-inch and 24-inch iMacs are great machines, except for the matter of those random freezing issues. We can all agree that a Mac, no matter how cool it is, becomes much less useful when it freezes up. Luckily, Apple has been fiendishly working on an update to fix this issue and the updates are ready for prime time. Notice the plural there, Apple has released a version for Tiger and a version for Leopard.
If you have one of the iMac that suffers from this particular ailment, get to downloading!
AppleiMac 27-inch with Retina 5K display (late 2014)