Windows, OS X, Linux oh my! Run all three together using VirtualBox by Sun Microsystems - Configure the Virtual Machine
Configure The Virtual Machine
Open VirtualBox and click "New" in the top menu pane. You will be prompted to fill out a registration form for a Sun Microsystems account. Once that's settled, name your virtual machine and specify the type of guest OS.
On the next page, select the amount of RAM that will be allocated to the virtual machine. It is recommended that you use at least 1GB = 1024MB for a Windows 7 installation. Note that the amount of memory your virtual machine uses is not dynamic. If you allocate 1GB of memory, then your virtual machine will be using 1GB of memory at all times. To prevent your native system from slowing to a complete crawl, VirtualBox limits the maximum amount of RAM you can select. Since I have 2GB of RAM on my laptop, VirtualBox capped off the memory usage of the guest OS to 1536 MB.
Now you will create the virtual disk image. This is where the guest will be installed and where all of its data will be stored. You have two choices when creating the image: fixed storage and dynamically expanding. Selecting fixed storage creates a disk image that is exactly as large as you specify from the very beginning, like a partition. Selecting dynamically expanding storage creates an empty image that can be filled with data over time. It is also much faster to create.
After choosing where you want the virtual disk image saved, it will be created with the file extension .vdi
VirtualBox will give you a quick summary of your virtual machine's stats. Click finish to advance to the main event.