Hi there -- and welcome!
Did you setup VirtualBox so that the virtual server has its own IP? Last time I tried VirtualBox & SME, things were very slow and didn't seem to work that well. That's why I use VMware now, but perhaps things have changed since then.
Craig
EDIT: Whoops, skipped right over the big chunk that has to do with networking... (Just skimming, really). That looks like an interesting way to test the server's gateway abilities, but wouldn't sme's dhcp function conflict with VirtualBox's internal dhcp server?