Ok, had to respond, because wow.
First off, the 50% poll thing is anything but accurate or scientific. A lot of people that would find this website, that thread, etc, would have been searching for resolutions to their problem of a freezing iMac, which is how I found this thread (I don't have a freezing iMac, but I was doing searches on the iMac for possible problems because I just ordered one).
Second, I have read from other threads that just running a fan control program of freezing iMac's that makes the fans kick in sooner has resolved, from what I saw of those complaining and their responses, 100% of the problem. It also seems to be that people not using the WiFi/Bluetooth chipsets dont have the issue. That doesn't make this problem ok, but if I just have to run an App that turns up my fans in order to make the much faster 4850 work just fine until Apple actually fixes it, thats hardly worth giving up a superior video card for.
Finally, To the guy that uses "lol" over and over in his posts and talks about just building a gaming machine and blah blah blah, man, just stop. I am typing this on a Q6600, 4GB, 22" Dell monitor, etc etc computer I built in April last year. Even with shopping around, NewEgg, rebates, all that, it still came out to be $1200, and that doesn't even include the OS or my mouse because I already had a mouse.
Build a computer that matches the iMac, that means a Wireless N network card, speakers, 24" IPS, not 22, and even then, of equal color, brightness, etc. Then add an Operating System, and at least $100 to the cost for a year warranty and phone support. Then add the keyboard, mouse, everything that is comparable that comes with the iMac.
Make it work for a significant amount less than the iMac. Oh and it has to look good too, because with the iMac you are talking an all in one, and not as someone else pointed out a large, loud, box sitting next to you.
No download windows from Torrent sites like a tool, none of that, actual retail stuff here. Yes, you can get the components and build it for less usually, and it might even be faster, but once you add the value of the form factor, the warranty, the OS and other software, there is no way and hell its worth the headache of building it yourself and dealing with DOA equipment, sending crap back if its bad, whatever, and this is coming from a long time computer builder that loves this stuff. There is a difference between doing the hobby because you love it, and being realistic.
And as far as the ATI goes, just get it, there is already a workaround (which we shouldn't have to do, but at least it exists) and even if its not as big a deal now, in two years when you don't already have to trade in your computer it will seem like the damn right choice.