I've been virtualized for a wee bit now, and agree with Ant Evans - Virtualbox makes it insanely easy. I usually have 1-2 Linux (Mint, Kubuntu, CentOS) servers running in virtual boxen off of a Win7 host on a Toshiba laptop. I did have to max out the RAM for stability when running multiple VMs, it's now running at 16GB and has run 4 VMs, (Plus Windows with SQL Server installed and Oracle installed on the centOS) without choking.
I also really like the networking options between the V.Box VMs allowing for private vLAN, public NAT, etc.; and multiple virtual NICs to tailor the networking of your Servers to your precise need.
For example: a Linux VM with Apache/PHP pulling data for the web from a SQL Server instance installed on the host, and then writing said data to a separate centOS VM with Oracle installed. (Yes, it was ugly, and I do realize *now* that there were faster/easier ways to move the data. But it was fast set-up and I didn't have to pay for conversion software.)
All on a Toshiba L50-class laptop.