The Performance Tuning Guidelines for Windows Server 2008 has been updated to include perf guidelines for virtualization servers. To calculate the host RAM = (VMRamRequirementMB x #ofVMs) + (#of VMs x 32MBVMOverhead ) + (512MBHost),
The guide says:
...the root partition must have sufficient memory (leave at least 512 MB available) to provide services such as I/O virtualization, snapshot, and management to support the child partitions.
A good standard for the memory overhead of each VM is 32 MB for the first 1 GB of virtual RAM plus another 8 MB for each additional GB of virtual RAM. This should be factored in the calculations of how many VMs to host on a physical server. The memory overhead varies depending on the actual load and amount of memory that is assigned to each VM.