There have been a lot of posts about the problem with the new ATI 4850 card. I just installed VMware Fusion and XP and had a total freeze. I had to shut down the iMac and restart. When I started the VM, it was totally corrupted and I had to do a reinstall of XP. I did find this on the VMware site which is just another problem with the 4850 version. I do not think it was the cause, but it is something to remember if you use Fusion.
"Enabling 3D acceleration on systems with new chipsets and running 3D applications within VMware Fusion might cause your virtual machine to fail
Enabling 3D acceleration on the Mac Pros and iMacs released in March 2009 that ship with the new ATI Radeon HD 4850 and 4870 chipsets might cause VMware Fusion to fail.
Workaround: Turn off 3D acceleration by deselecting Accelerate 3D Graphics in Virtual Machine > Settings > Display when the virtual machine is powered off."
"Enabling 3D acceleration on systems with new chipsets and running 3D applications within VMware Fusion might cause your virtual machine to fail
Enabling 3D acceleration on the Mac Pros and iMacs released in March 2009 that ship with the new ATI Radeon HD 4850 and 4870 chipsets might cause VMware Fusion to fail.
Workaround: Turn off 3D acceleration by deselecting Accelerate 3D Graphics in Virtual Machine > Settings > Display when the virtual machine is powered off."