I'm running ESXi 4.0.0 on a whitebox with a Gigabyte motherboard. I run 3 XP Professional guest OS VMs. I've been running great with no problems for months with 4G ram. Recently I added another 4G to the machine to bring the total to 8G. After restarting the VMs they will run for an hour or two and then restart with a Bugcheck error: 0x0000004e (0x00000099, 0x00000000, 0x00000000, 0x00000000). I took the 4G back out and no more problems.