I can reproduce the ATI 4850 freeze by using the iTunes visualizer in fullscreen mode. So I wanted to see if this could help find the problem, and it did. I used the OpenGL Profiler that's included with the dev tools. I attached the profiler to the iTunes process, and dumped all the OpenGL calls. iTunes eventually crashed the profiler, leaving my computer in a state of limbo (not completely frozen... could still use expose to see what was on my desktop, but otherwise the computer was completely locked up.... but the profiler 'trapped' the iTunes process, which stopped it from completely freezing my machine). Sent a bug report to Apple. The problem is the OpenGL implementation in the ATI driver. I've tried to freeze my computer under Win XP in Boot Camp by playing Portal with the highest settings. No freezes, but all the Valve games use DirectX, not OpenGL. And yes, the version of OS X that ships with the iMac is a custom build. I now have a few applications that won't launch properly on my iMac, but work on my MacBook Pro with the exact same (major) build of OS X. The only difference is the hardware. There might be some CoreAudio conflicts with these machines as well. I'm debating waiting for a firmware update, or just downgrading to the default NVIDIA iMac. Thanks.