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.
Here are some recommended links talking about performance tuning and right sizing when planning a deployment