Installed VirtualBox on Ubuntu x64. At first there was a problem: starting Guest hung system. But this problem was eventually found as a BIOS setting. Avoid enabling Limit CPUID MaxVal in BIOS when using VMM. Now system is working fine. At first, VBox installed from Ubuntu PPA, but then removed that and installed as per this page. Either would probably work fine since problem was BIOS setting.
Now have Ubuntu x86 and x86 guests running on x64 system.
Clean systems are now available for testing programs in both x86 and x64.