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The Opmantek Virtual Appliance ships with the virtual machine hardware version set very low for maximum compatibility, but you may want to upgrade the hardware version to a higher level (to leverage performance benefits, remove memory limitations, etc.) as described on the VMware website.
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VirtualBox will import directly from an OVF file so you can just fire start up VirtualBox and select "Import Appliance..."
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It should be safe to Import. The import process took may take a couple of minutes to complete, just enough time to grab a cup of coffee I'd say.
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