It's hard to say.
It depends on how much I/O you'll do on it.
If you set the guests's disk as read only you should use less bandwidth.
The guest disk would be r/w and the disk usage would be low (I think), in fact it's to test new services like bind, nis, apache, samba, cups, mysql... install, configure, test how they work
The question could be rewritten, perhaps: Â¿How much network overhead do we have due to guest-host normal interactions?