Will you be virtualizing the machine hosting SQL Monitor itself, or the entities it monitors? Either or both is fine, though there are a couple points you may wish to be aware of.
SQL Monitor's licensing uses a machine hash to identify the hardware (or virtual hardware) that the software was activated on (just the base monitor itself, the monitored machines can change as much as you like). Adding RAM or moving about the instance the base monitor is installed on shouldn't invalidate this, though if you are concerned you can deactivate / activate the licence around the VM operation.
The data repository SQL Monitor uses can become busy when you're monitoring a lot of machines and as such, probably doesn't want to be on a virtual disk (pass-through or SAN is preferred).
As for the monitored entities, there should not no issues relating to virtualization.