People who want to start with Hyper-V and virtualization, often asks how much memory they need on their physical server. The answer is: What is the length of a fish.
A key to a successful Hyper-V implementation is adding enough memory to your hosts.
Assigning your VMs with ‘enough’ RAM, will decrease the I/O for your storage, and give you a more responsive environment. You could basically have all the CPU power in the world, but the VMs would not take advantage of that (would be unused) if you don’t have enough memory when running your VMs at the same time.
1) How many VMs will be running, and how much RAM will be allocated ?
Plan this carefully, and also keep in mind that SP1 supports allocating more RAM than actually installed on the host so you could over commit your server
2) Workloads within your VMs
A VM with SQL installed, is the same as a physical machine with SQL installed. It require memory, memory, and memory.
3) Live Migration / Quick Migrations
If your physical server(s) is a part of a Failover Cluster, keep in mind that in case of a failover, you need sufficient resources to support the additional VMs. In worst case, the VMs would not be able to start after the failover.