I have recently done a yum upgrade on an oracle linux 6.2 vm image which was a desktop install to get it up to 6.4 latest. This image is running on vm server 3.0.3).
Once the update had completed and rebooted the desktop/graphical login screen is totally corrupted and am unable to do much with it so had to go back to a previous snapshot.
I have also tried this on a VM 3.2.2 setup, Created a Oracle Linux 6.3 guest and done a yum update to bring it up to the latest 6.4. Once rebooted the desktop is totally corrupted on this as well.