I had some trouble with Parallels, it did not start any Virtual OS. But it's solved now - if you have same trouble, remember to check Intel VT or Intel Virtualization Technics settings from BIOS
[<- when you are a n00b, you can only blame yourself]
This topic was "Post your virtualization screenshot" .... well, im sure someone has figured out this before, but :
Set up Spaces, and you can have Virtual OS on other Desktop as "Full" and you can switch between them with F8.
like in my photo...
Just open Paralles, and first OS. Push F8 and drag it to space2. Then open second, drag it to 3 and third...... Start them and use Full Screen mode and you have 3 - 4 os running, and by pushing F8 you switch
and as it seems, 3 OS running leaves some RAM free.. this is, if you - by some odd reason - need Windoze for something
Of course you can use Coherence, but i prefer this.