Virtualbox is the easiest to use. It doesn't get much simpler. However,Ubuntu
has gone with KVM. See: https://help.ubuntu.com/community/KVM

This is kernel virtualization. There is also Xen which uses a similar approach,
but it is no longer officially supported by Ubuntu. You can do it yourself
though because everything that you need is in the repos. See:
https://help.ubuntu.com/community/Xen

Then there is qemu. See:
https://help.ubuntu.com/community/Installation/QemuEmulator

I have used qemu and did not find it easy to use and when I got it to work, I
was not happy with the results. i.e. not worth the effort IMO. Qemu is an
emulator and not a virtualization tool and as such performance is not as good. I
believe that KVM uses Qemu as part of its virtualization to emulate various
cores, but am no expert.

I don't have an AMD 64 single core so I have not used KVM or Xen. I have tried,
but failed. My processor is not good enough.

There are several other VM managers including VMWare Server (free, but
proprietary), VMWare Workstation (commercial), Parallels (commercial), Virtual
Iron, (not sure), etc. See the tables here for a comparison:
en....pedia.org/.../Comparison_of_platform_virtual_machines

I run VirtualBox and it runs fine provided you control the amount of RAM and
give enough RAM and video (I use 640 MBs of RAM and 64 MBs of video). One side
benefit is that you can use it in seamelss mode and get two panels, one for
guest and one for host, on the same shared desktop.