I don't know why but the system requirements for sourcefire's virtual sensor is burried in the wording of their datasheets... anyway saved here so ease of reference :)

Sensor

VMware ESX/ESXi 4.1/5.0 or Xen 3.3.2/3.4.2
at least one CPU
min 1GB RAM

Defence Centre

VMware ESX/ESXi 4.1/5.0 or Xen 3.3.2/3.4.2
at lest two CPU
min 2GB RAM

...hum, I wonder if they're boot in virtualbox? ;)

[UPDATE]
The sensor provisions 50Gb of disk, and the manager 250Gb, the OVF supports thin disk provisioning so you don't need all that storage on day1.