I have a dual-core machine with the host OS and many guest virtual OSs. Although I have 8GB of RAM, I notice a slowdown when I turn some virtual machine on (and it takes only 1GB RAM).

I was told that I should move virtual machine hard disk file to a separate (another) physical hard drive in my PC to get better performance. This way the head of the hard disk would not have to jump from the virtual OS to the host OS as each hard drive would have its own head to deal with the OS: hard drive 1 head for host OS and hard drive 2 head for guest OS. 

Is this true? Should I get another hard disk only for virtual machine hard disk files?