I really liked VirtualBox and got a lot of use out of it when running my old Windows XP system under Ubuntu. However, the lack of support for 64-bit systems proved too problematic. In testing Ubuntu 8.04 (Hardy Heron) I was only interested in testing the 64-bit versions as that was all I would be running. As some bugs are architecture dependent (iproute and monsterz come to mind) it's important to perform testing on the architecture you will actually use. Under Ubuntu 7.10 (Gutsy Gibbon) I had some issues with QEMU and KVM though I was able to use them enough to do testing (though often only through QEMU with specific parameters at a less than ideal speed). However in Ubuntu 8.04 I found both applications improved and decided to search for a nice front-end to replace the functionality I was used to with VirtualBox.

Turns out that VirtualBox doesn't support the Ubuntu Server Linux kernel due to it's reliance on PAE support (even in 64-bit apparently). The solution is to run the generic kernel instead. I'm not sure if that can be done when installing Ubuntu Server, or if one has to install Ubuntu desktop and then strip it down. I'll have to do some testing when I get the time.

For more information about what guest operating systems are supported in virtual box:
http://www.virtualbox.org/wiki/Guest_OSes

Since I couldn't get the amd64 version of Ubuntu running under VirtualBox OSE I tried the i386 version. The installation went fine but when rebooting into the system I got this message, "PANIC: CPU too old for this kernel." I was hoping to have Ubuntu running under VirtualBox to spare me from always using my system for tests. It looks like it might take some research to accomplish that goal.

I just tried to install a 64-bit OS (Ubuntu Server) under VirtualBox OSE 1.5.0 and received the message, "Your CPU does not support long mode. Use a 32bit distribution. I tried toggling the "Enable VT-x/AMD-V" setting but that didn't help. I haven't investigated this yet but I will be disappointed if this is a limitation of VirtualBox OSE.