The answer is pretty simple,
All needed to be done to make my setup work is to set the Guest NIC to NAT.
VirtualBox has its own Router-alike function, Guest VM receives IP from VBox DHCP and network is working.
Now I wanted to port-forward 3389 to use Guest's RDP protocol,
This is how it's done -
http://sk.c-wd.net/wp/2008/08/17/vir...-windows-host/
All works,
/me happy!
-- Cheers