Yes you can run Windows 8 on a Windows 7 Computer using Virtual Machine Software. Depending on your Hardware configuration, you can choose the amount of Memory you want Windows 8 to have. Running another Operating System inside of a Virtual Machine requires you to have good computer configuration. Whether you wish to run Windows 8 on a Desktop or Laptop running Windows 7, actual hardware configuration matters as the resources of your hardware are going to be shared by both the operating systems.
In order to run Windows 8 on your Windows 7 Computer, you can use the best Virtual Machine Software named as Oracle VM VirtualBox. Virtual Computers are often used by software developers to test 3rd party software applications on other operating systems. When running Windows 8 on a Virtual Computer, you have absolute flexibility of selection of multiple hardware configurations. You can have Multiple Virtual Hard Disks, CD / DVD ROM and even Floppy drive.
One of the best advantages of running Windows 8 on Virtual Computer is that you can have single or multiple monitors attached to your virtual Windows 8 computer. Separate Popup Windows will open up providing you separate monitor access to your multiple monitors to check the Multiple Monitor features of Windows 8 like taskbar, multiple monitor wallpaper, etc.
The above screenshot displays quick way to create a virtual pc for 32 bit or 64 bit Windows 8 computer. Yes there are other operating systems supported as well and you can run other Microsoft Windows and other Operating Systems as well. All you need to run multiple operating systems (like Windows 8 on Windows 7 as displayed in this blog post) on a single computer is strong enough hardware possibly with maximum possible RAM, Hardware Virtualization Support, Monitor with big resolution, etc.