[ubuntu] 'configuring' multiple VMs

Hi all!
I'm posting this thread after hard hitting ma head for hours on configuring NETWORKING on my VM network (through NAT). Also, I wanna know that do I need a "workstation" version of VMware or VirtualBox to have ALL my guest OS systems in ma virtual machine which I'm hosting on Ubuntu 12.04, BOOT TOGETHER.

I know that what I wrote above would get a bit choppy for you guys so let's get it straight-

1. Do I need a "workstation" version of VMware or VirtualBox or any other VM program you recommend, to have all my "guest" operating systems and they're-
* Windows XP SP2
* Windows 7 Professional
* Ubuntu
* Kubuntu, boot together and "I can switch to them ON FLY".

2. How to configure "networking" in all guest OS? Such that, each VM system should be able to PING each other and my HOST system too .i.e. Ubuntu 12.04 at which the VM is hosted.

3. Do the Open-Source version of VMware workstation would solve my problem? As I don't wanna buy it. I <3 open source and also a supporter of it. :p

My system specifications are-
CPU: AMD FX-4100 Quad-Core Bulldozer 3.6 Ghz
GPU: ATI Radeon HD 4250 512 MB
MOBO: Gigabyte GA-880GM-D2H
RAM: 4 GB DDR3 Kingston
HDD: 1 TB Western Digit 7200 Rpm

What say?