If you plan to run multiple virtual machines, then giving a virtual machine 2.7 GB of RAM out of a total of 6 GB is probably too much. I give most of my virtual machines, including Ubuntu virtual machines, 1 GB of RAM, even when I have enough physical RAM available that I could allocate more. However, depending on what you plan to do with your Ubuntu system, it probably doesn't need 2.7 GB of RAM. If you're running a single virtual machine, the suggestion you read is OK (provided that your host machine has enough RAM, which it does). You should give virtual machine however much RAM you need for the tasks you're using it to perform.